Best way to test for a variable s existence in PHP isset is clearly broken

Question

From the isset   docs   isset   will return FALSE if testing a variable that has been set to NULL    Basically  isset   doesn t check for whether the variable is set at all  but whether it s set to anything but NULL   Given that  what s the best way to actually check for the existence of a variable   I tried something like   if isset  v      is null  v      the   is necessary to avoid the warning when  v is not set  but is null   has a similar problem to isset    it returns TRUE on unset variables   It also appears that      v     NULL    works exactly like  is null  v   so that s out  too   How are we supposed to reliably check for the existence of a variable in PHP     Edit  there is clearly a difference in PHP between variables that are not set  and variables that are set to NULL    lt  php  a   array  b    gt  NULL   var dump  a     PHP shows that  a  b   exists  and has a NULL value   If you add   var dump isset  a  b      var dump isset  a  c        you can see the ambiguity I m talking about with the isset   function   Here s the output of all three of these var dump  s   array 1        b    gt    NULL   bool false  bool false      Further edit  two things   One  a use case   An array being turned into the data of an SQL UPDATE statement  where the array s keys are the table s columns  and the array s values are the values to be applied to each column   Any of the table s columns can hold a NULL value  signified by passing a NULL value in the array   You need a way to differentiate between an array key not existing  and an array s value being set to NULL  that s the difference between not updating the column s value and updating the column s value to NULL   Second  Zoredache s answer  array key exists   works correctly  for my above use case and for any global variables    lt  php  a   NULL  var dump array key exists  a    GLOBALS    var dump array key exists  b    GLOBALS      outputs   bool true  bool false    Since that properly handles just about everywhere I can see there being any ambiguity between variables that don t exist and variables that are set to NULL  I m calling array key exists   the official easiest way in PHP to truly check for the existence of a variable    Only other case I can think of is for class properties  for which there s property exists    which  according to its docs  works similarly to array key exists   in that it properly distinguishes between not being set and being set to NULL

User · Answer

I have to say in all my years of PHP programming, I have never encountered a problem with isset() returning false on a null variable. OTOH, I have encountered problems with isset() failing on a null array entry - but array_key_exists() works correctly in that case.

For some comparison, Icon explicitly defines an unused variable as returning &null so you use the is-null test in Icon to also check for an unset variable. This does make things easier. On the other hand, Visual BASIC has multiple states for a variable that doesn't have a value (Null, Empty, Nothing, ...), and you often have to check for more than one of them. This is known to be a source of bugs.

User · Answer

I don t agree with your reasoning about NULL  and saying that you need to change your mindset about NULL is just weird   I think isset   was not designed correctly  isset   should tell you if the variable has been set and it should not be concerned with the actual value of the variable   What if you are checking values returned from a database and one of the columns have a NULL value  you still want to know if it exists even if the value is NULL   nope dont trust isset   here   likewise    a   array   test    gt  1   hello    gt  NULL    var dump isset  a  test           TRUE var dump isset  a  foo            FALSE var dump isset  a  hello          FALSE   isset   should have been designed to work like this   if isset  var   amp  amp   var   NULL         this way we leave it up to the programmer to check types and not leave it up to isset   to assume its not there because the value is NULL - its just stupid design

User · Answer

As an addition to greatbigmassive s discussion of what NULL means  consider what  the existence of a variable  actually means   In many languages  you have to explicitly declare every variable before you use it  this may determine its type  but more importantly it declares its scope  A variable  exists  everywhere in its scope  and nowhere outside it - be that a whole function  or a single  block    Within its scope  a variable assigns some meaning to a label which you  the programmer  have chosen  Outside its scope  that label is meaningless  whether you use the same label in a different scope is basically irrelevant    In PHP  variables do not need to be declared - they come to life as soon as you need them  When you write to a variable for the first time  PHP allocates an entry in memory for that variable  If you read from a variable that doesn t currently have an entry  PHP considers that variable to have the value NULL   However  automatic code quality detectors will generally warn you if you use a variable without  initialising  it first  Firstly  this helps detect typos  such as assigning to  thingId but reading from  thing id  but secondly  it forces you to consider the scope over which that variable has meaning  just as a declaration would   Any code that cares whether a variable  exists  is part of the scope of that variable - whether or not it has been initialised  you as a programmer have given that label meaning at that point of the code  Since you re using it  it must in some sense  exist   and if it exists  it must have an implicit value  in PHP  that implicit value is null   Because of the way PHP works  it is possible to write code that treats the namespace of existent variables not as a scope of labels you have given meaning to  but as some kind of key-value store  You can  for instance  run code like this   var     GET  var name      var     GET  var value     Just because you can  doesn t mean it s a good idea   It turns out  PHP has a much better way of representing key-value stores  called associative arrays  And although the values of an array can be treated like variables  you can also perform operations on the array as a whole  If you have an associative array  you can test if it contains a key using array key exists      You can also use objects in a similar way  dynamically setting properties  in which case you can use property exists   in exactly the same way  Of course  if you define a class  you can declare which properties it has - you can even choose between public  private  and protected scope    Although there is a technical difference between a variable  as opposed to an array key  or an object property  that hasn t been initialised  or that has been explicitly unset    and one whose value is null  any code that considers that difference to be meaningful is using variables in a way they re not meant to be used

User · Answer

If I run the following   echo   lt  php echo  foo    gt     php   I get an error   PHP Notice   Undefined variable  foo in  home altern8 - on line 1   If I run the following   echo   lt  php if   isset  foo      echo  foo      gt     php   I do not get the error    If I have a variable that should be set  I usually do something like the following    foo   isset  foo     foo   null    or  if     isset  foo     foo   null    That way  later in the script  I can safely use  foo and know that it  is set   and that it defaults to null  Later I can if   is null  foo                  if I need to and know for certain that the variable exists  even if it is null   The full isset documentation reads a little more than just what was initially pasted  Yes  it returns false for a variable that was previously set but is now null  but it also returns false if a variable has not yet been set  ever  and for any variable that has been marked as unset  It also notes that the NULL byte    0   is not considered null and will return true      Determine whether a variable is set       If a variable has been unset with   unset    it will no longer be set    isset   will return FALSE if testing a   variable that has been set to NULL    Also note that a NULL byte    0   is   not equivalent to the PHP NULL   constant

User · Answer

Try using  unset  v    It seems the only time a variable is not set is when it is specifically unset  v   It sounds like your meaning of  existence  is different than PHP s definition  NULL is certainly existing  it is NULL

User · Answer

Attempting to give an overview of the various discussions and answers   There is no single answer to the question which can replace all the ways isset can be used  Some use cases are addressed by other functions  while others do not stand up to scrutiny  or have dubious value beyond code golf  Far from being  broken  or  inconsistent   other use cases demonstrate why isset s reaction to null is the logical behaviour   Real use cases  with solutions   1  Array keys  Arrays can be treated like collections of variables  with unset and isset treating them as though they were  However  since they can be iterated  counted  etc  a missing value is not the same as one whose value is null   The answer in this case  is to use array key exists   instead of isset     Since this is takes the array to check as a function argument  PHP will still raise  notices  if the array itself doesn t exist  In some cases  it can validly be argued that each dimension should have been initialised first  so the notice is doing its job  For other cases  a  recursive  array key exists function  which checked each dimension of the array in turn  would avoid this  but would basically be the same as  array key exists  It is also somewhat tangential to the handling of null values   2  Object properties  In the traditional theory of  Object-Oriented Programming   encapsulation and polymorphism are key properties of objects  in a class-based OOP implementation like PHP s  the encapsulated properties are declared as part of the class definition  and given access levels  public  protected  or private     However  PHP also allows you to dynamically add properties to an object  like you would keys to an array  and some people use class-less objects  technically  instances of the built in stdClass  which has no methods or private functionality  in a similar way to associative arrays  This leads to situations where a function may want to know if a particular property has been added to the object given to it   As with array keys  a solution for checking object properties is included in the language  called  reasonably enough  property exists   Non-justifiable use cases  with discussion  3  register globals  and other pollution of the global namespace  The register globals feature added variables to the global scope whose names were determined by aspects of the HTTP request  GET and POST parameters  and cookies   This can lead to buggy and insecure code  which is why it has been disabled by default since PHP 4 2  released Aug 2000 and removed completely in PHP 5 4  released Mar 2012  However  it s possible that some systems are still running with this feature enabled or emulated  It s also possible to  pollute  the global namespace in other ways  using the global keyword  or  GLOBALS array   Firstly  register globals itself is unlikely to unexpectedly produce a null variable  since the GET  POST  and cookie values will always be strings  with    still returning true from isset   and variables in the session should be entirely under the programmer s control   Secondly  pollution of a variable with the value null is only an issue if this over-writes some previous initialization   Over-writing  an uninitialized variable with null would only be problematic if code somewhere else was distinguishing between the two states  so on its own this possibility is an argument against making such a distinction   4  get defined vars and compact  A few rarely-used functions in PHP  such as get defined vars and compact  allow you to treat variable names as though they were keys in an array  For global variables  the super-global array  GLOBALS allows similar access  and is more common  These methods of access will behave differently if a variable is not defined in the relevant scope   Once you ve decided to treat a set of variables as an array using one of these mechanisms  you can do all the same operations on it as on any normal array  Consequently  see 1   Functionality that existed only to predict how these functions are about to behave  e g   will there be a key  foo  in the array returned by get defined vars    is superfluous  since you can simply run the function and find out with no ill effects   4a  Variable variables    foo   While not quite the same as functions which turn a set of variables into an associative array  most cases using  variable variables    assign to a variable named based on this other variable   can and should be changed to use an associative array instead    A variable name  fundamentally  is the label given to a value by the programmer  if you re determining it at run-time  it s not really a label but a key in some key-value store  More practically  by not using an array  you are losing the ability to count  iterate  etc  it can also become impossible to have a variable  outside  the key-value store  since it might be over-written by   foo   Once changed to use an associative array  the code will be amenable to solution 1  Indirect object property access  e g   foo- gt  property name  can be addressed with solution 2   5  isset is so much easier to type than array key exists  I m not sure this is really relevant  but yes  PHP s function names can be pretty long-winded and inconsistent sometimes  Apparently  pre-historic versions of PHP used a function name s length as a hash key  so Rasmus deliberately made up function names like htmlspecialchars so they would have an unusual number of characters     Still  at least we re not writing Java  eh      6  Uninitialized variables have a type  The manual page on variable basics includes this statement      Uninitialized variables have a default value of their type depending on the context in which they are used   I m not sure whether there is some notion in the Zend Engine of  uninitialized but known type  or whether this is reading too much into the statement   What is clear is that it makes no practical difference to their behaviour  since the behaviours described on that page for uninitialized variables are identical to the behaviour of a variable whose value is null  To pick one example  both  a and  b in this code will end up as the integer 42   unset  a    a    42    b   null   b    42     The first will raise a notice about an undeclared variable  in an attempt to make you write better code  but it won t make any difference to how the code actually runs    99  Detecting if a function has run   Keeping this one last  as it s much longer than the others  Maybe I ll edit it down later      Consider the following code    test value    hello   foreach    list of things as  thing         if   some test  thing   test value               result   some function  thing           if   isset  result          echo  The test passed at least once        If some function can return null  there s a possibility that the echo won t be reached even though some test returned true  The programmer s intention was to detect when  result had never been set  but PHP does not allow them to do so   However  there are other problems with this approach  which become clear if you add an outer loop   foreach    list of tests as  test value            something s missing here        foreach    list of things as  thing             if   some test  thing   test value                   result   some function  thing                       if   isset  result              echo  The test passed at least once              Because  result is never initialized explicitly  it will take on a value when the very first test passes  making it impossible to tell whether subsequent tests passed or not  This is actually an extremely common bug when variables aren t initialised properly   To fix this  we need to do something on the line where I ve commented that something s missing  The most obvious solution is to set  result to a  terminal value  that some function can never return  if this is null  then the rest of the code will work fine  If there is no natural candidate for a terminal value because some function has an extremely unpredictable return type  which is probably a bad sign in itself   then an additional boolean value  e g   found  could be used instead   Thought experiment one  the very null constant  PHP could theoretically provide a special constant - as well as null - for use as a terminal value here  presumably  it would be illegal to return this from a function  or it would be coerced to null  and the same would probably apply to passing it in as a function argument  That would make this very specific case slightly simpler  but as soon as you decided to re-factor the code - for instance  to put the inner loop into a separate function - it would become useless  If the constant could be passed between functions  you could not guarantee that some function would not return it  so it would no longer be useful as a universal terminal value   The argument for detecting uninitialised variables in this case boils down to the argument for that special constant  if you replace the comment with unset  result   and treat that differently from  result   null  you are introducing a  value  for  result that cannot be passed around  and can only be detected by specific built-in functions   Thought experiment two  assignment counter  Another way of thinking about what the last if is asking is  has anything made an assignment to  result   Rather than considering it to be a special value of  result  you could maybe think of this as  metadata  about the variable  a bit like Perl s  variable tainting   So rather than isset you might call it has been assigned to  and rather than unset  reset assignment state   But if so  why stop at a boolean  What if you want to know how many times the test passed  you could simply extend your metadata to an integer and have get assignment count and reset assignment count     Obviously  adding such a feature would have a trade-off in complexity and performance of the language  so it would need to be carefully weighed against its expected usefulness  As with a very null constant  it would be useful only in very narrow circumstances  and would be similarly resistant to re-factoring    The hopefully-obvious question is why the PHP runtime engine should assume in advance that you want to keep track of such things  rather than leaving you to do it explicitly  using normal code

User · Answer

THE only way to know if a variable is defined in current scope   GLOBALS is not trustworthy  is array key exists   var name   get defined vars

User · Answer

According to the PHP Manual for the empty   function   Determine whether a variable is considered to be empty  A variable is considered empty IF IT DOES NOT EXIST or if its value equals FALSE  empty   does not generate a warning if the variable does not exist     My emphasis    That means the empty   function should qualify as the  best way to test a variable s existence in PHP   per the title Question   However  this is not good enough  because the empty   function can be fooled by a variable that does exist and is set to NULL   I m interrupting my earlier answer to present something better  because it is less cumbersome than my original answer  which follows this interruption  for comparing      function undef  dnc    do not care what we receive      inf ob get contents                  get the content of the buffer     ob end clean                          stop buffering outputs  and empty the buffer     if  inf gt                               if test associated with the call to this function had an output       if false   strpos  inf   Undef      if the word  Undefined  was part of the output         return true                       tested variable is undefined           return false                          tested variable is not undefined       Two simple lines of code can use the above function to reveal if a variable is undefined     ob start                                pass all output messages  including errors  to a buffer   if undef  testvar   null                in this case the variable being tested is  testvar   You can follow those two lines with anything appropriate  such as this example       echo  variable is undefined      else     echo  variable exists  holding some value      I wanted to put the call to ob start   and the   testvar   null  inside the function  and simply pass the variable to the function  but it doesn t work   Even if you try to use  pass by reference  of the variable to the function  the variable BECOMES defined  and then the function can never detect that it previously had been undefined   What is presented here is a compromise between what I wanted to do  and what actually works   The preceding implies that there is another way to always avoid running into the  Undefined variable  error message    The assumption here is  preventing such a message is why you want to test to see if a variable is undefined       function inst  amp  v    return       receive any variable passed by reference  instantiates the undefined   Just call that function before doing something to your  testvar      inst  testvar                    The function doesn t affect any value of any already-existing variable   The newly-instantiated variable s value is set to null  of course    Interruption ends   So  after some studying and experimenting  here is something guaranteed to work    function myHndlr  en   es   ef   el     global  er      er    substr  es  0  18      Undefined variable       return        er   false   if empty  testvar      set error handler  myHndlr         testvar     null      restore error handler        if  er      will be 1  true  if the tested variable was not defined         do whatever you think is appropriate to the undefined variable      The explanation  A variable  er is initialized to a default value of  no error    A  handler function  is defined   If the  testvar  the variable we want to know whether or not is undefined  passes the preliminary empty   function test  then we do the more thorough test   We call the set error handler   function to use the previously-defined handler function   Then we do a simple identity-comparison involving  testvar  WHICH IF UNDEFINED WILL TRIGGER AN ERROR   The handler function captures the error and specifically tests to see if the reason for the error is the fact that the variable is undefined   The result is placed in the error-information variable  er  which we can later test to do whatever we want as a result of knowing for sure whether or not  testvar was defined   Because we only need the handler function for this limited purpose  we restore the original error-handling function   The  myHndlr  function only needs to be declared once  the other code can be copied to whatever places are appropriate  for  testvar or any other variable we want to test this way

User · Answer

Object properties can be checked for existence by property exists  Example from a unit test   function testPropertiesExist          sl   amp   this- gt system log       props   array  log id                       type                       message                       username                       ip address                       date added         foreach  props as  prop             this- gt assertTrue property exists  sl   prop                               Property  lt   prop  gt  exists

User · Answer

If the variable you are checking would be in the global scope you could do   array key exists  v    GLOBALS

User · Answer

Object properties can be checked for existence by property exists  Example from a unit test   function testPropertiesExist          sl   amp   this- gt system log       props   array  log id                       type                       message                       username                       ip address                       date added         foreach  props as  prop             this- gt assertTrue property exists  sl   prop                               Property  lt   prop  gt  exists

User · Answer

You can use the compact language construct to test for the existence of a null variable  Variables that do not exist will not turn up in the result  while null values will show    x   null   y    y     r   compact  x    y    z    print r  r       Output     Array         x    gt        y    gt  y          In the case of your example   if  compact  v            True if  v exists  even when null         False on var  v  without assignment and when  v does not exist      Of course for variables in global scope you can also use array key exists     B t w  personally I would avoid situations like the plague where there is a semantic difference between a variable not existing and the variable having a null value  PHP and most other languages just does not think there is

User · Answer

You can use the compact language construct to test for the existence of a null variable  Variables that do not exist will not turn up in the result  while null values will show    x   null   y    y     r   compact  x    y    z    print r  r       Output     Array         x    gt        y    gt  y          In the case of your example   if  compact  v            True if  v exists  even when null         False on var  v  without assignment and when  v does not exist      Of course for variables in global scope you can also use array key exists     B t w  personally I would avoid situations like the plague where there is a semantic difference between a variable not existing and the variable having a null value  PHP and most other languages just does not think there is

User · Answer

Explaining NULL  logically thinking  I guess the obvious answer to all of this is    Don t initialise your variables as NULL  initalise them as something relevant to what they are intended to become   Treat NULL properly  NULL should be treated as  non-existant value   which is the meaning of NULL  The variable can t be classed as existing to PHP because it hasn t been told what type of entity it is trying to be  It may aswell not exist  so PHP just says  Fine  it doesn t because there s no point to it anyway and NULL is my way of saying this    An argument  Let s argue now   But NULL is like saying 0 or FALSE or      Wrong  0-FALSE-   are all still classed as empty values  but they ARE specified as some type of value or pre-determined answer to a question  FALSE is the answer to yes or no    is the answer to the title someone submitted  and 0 is the answer to quantity or time etc  They ARE set as some type of answer result which makes them valid as being set   NULL is just no answer what so ever  it doesn t tell us yes or no and it doesn t tell us the time and it doesn t tell us a blank string got submitted  That s the basic logic in understanding NULL   Summary  It s not about creating wacky functions to get around the problem  it s just changing the way your brain looks at NULL  If it s NULL  assume it s not set as anything  If you are pre-defining variables then pre-define them as 0  FALSE or    depending on the type of use you intend for them   Feel free to quote this  It s off the top of my logical head

User · Answer

I think the only full solution is to report notices with  error reporting E ALL      Enables E NOTICE   But you will have to fix all the notices generated by undefined variables  constants  array keys  class properties amongst others  Once you have done that you won t have to worry about the difference between null and not declared variables  and the ambiguity dissappears   Enabling notice reporting might not be a good alternative in all situations  but there are good reasons to enable it   Why should I fix E NOTICE errors   In my case was more than a year working in a proyect without it  but was used to be careful about declaring variables  so it was fast to transition

User · Answer

If I run the following   echo   lt  php echo  foo    gt     php   I get an error   PHP Notice   Undefined variable  foo in  home altern8 - on line 1   If I run the following   echo   lt  php if   isset  foo      echo  foo      gt     php   I do not get the error    If I have a variable that should be set  I usually do something like the following    foo   isset  foo     foo   null    or  if     isset  foo     foo   null    That way  later in the script  I can safely use  foo and know that it  is set   and that it defaults to null  Later I can if   is null  foo                  if I need to and know for certain that the variable exists  even if it is null   The full isset documentation reads a little more than just what was initially pasted  Yes  it returns false for a variable that was previously set but is now null  but it also returns false if a variable has not yet been set  ever  and for any variable that has been marked as unset  It also notes that the NULL byte    0   is not considered null and will return true      Determine whether a variable is set       If a variable has been unset with   unset    it will no longer be set    isset   will return FALSE if testing a   variable that has been set to NULL    Also note that a NULL byte    0   is   not equivalent to the PHP NULL   constant

User · Answer

I prefer using not empty as the best method to check for the existence of a variable that a  exists  and b  is not null   if   empty  variable   do something

User · Answer

If the variable you are checking would be in the global scope you could do   array key exists  v    GLOBALS

User · Answer

isset checks if the variable is set and  if so  whether its value is not NULL  The latter part is  in my opinion  not within the scope of this function  There is no decent workaround to determine whether a variable is NULL because it is not set or because it is explicitly set to NULL   Here is one possible solution    e1   error get last     isNULL   is null   x    e2   error get last     isNOTSET    e1     e2  echo sprintf  isNOTSET   d  isNULL   d    isNOTSET   isNULL       Sample output     when  x is not set  isNOTSET  1  isNULL  1    when  x   NULL      isNOTSET  0  isNULL  1    when  x   false     isNOTSET  0  isNULL  0   Other workaround is to probe the output of get defined vars      vars   get defined vars     isNOTSET    array key exists  x    vars    isNULL    isNOTSET   true   is null  x   echo sprintf  isNOTSET   d  isNULL   d    isNOTSET   isNULL       Sample output     when  x is not set  isNOTSET  1  isNULL  1    when  x   NULL      isNOTSET  0  isNULL  1    when  x   false     isNOTSET  0  isNULL  0

User · Answer

Try using  unset  v    It seems the only time a variable is not set is when it is specifically unset  v   It sounds like your meaning of  existence  is different than PHP s definition  NULL is certainly existing  it is NULL

User · Answer

I have to say in all my years of PHP programming, I have never encountered a problem with isset() returning false on a null variable. OTOH, I have encountered problems with isset() failing on a null array entry - but array_key_exists() works correctly in that case.

For some comparison, Icon explicitly defines an unused variable as returning &null so you use the is-null test in Icon to also check for an unset variable. This does make things easier. On the other hand, Visual BASIC has multiple states for a variable that doesn't have a value (Null, Empty, Nothing, ...), and you often have to check for more than one of them. This is known to be a source of bugs.

User · Answer

Object properties can be checked for existence by property exists  Example from a unit test   function testPropertiesExist          sl   amp   this- gt system log       props   array  log id                       type                       message                       username                       ip address                       date added         foreach  props as  prop             this- gt assertTrue property exists  sl   prop                               Property  lt   prop  gt  exists

User · Answer

Sometimes I get a little lost trying to figure out which comparison operation to use in a given situation  isset   only applies to uninitialized or explicitly null values  Passing assigning null is a great way to ensure a logical comparison works as expected   Still  it s a little difficult to think about so here s a simple matrix comparing how different values will be evaluated by different operations                    null   is null   isset   empty   if else   ternary   count gt 0     -----       -----     -----     -----   -----   -----     -----     -----        a          true      true              true                                    null        true      true              true                                                                    true    true                                    0                               true    true                        true                                        true    true                        true        1                               true            true      true      true        -1                              true            true      true      true                                        true            true      true      true         str                            true            true      true      true         0 1                            true            true      true      true        new Class                       true            true      true      true        To fit the table I compressed the labels a bit     a  refers to a declared but unassigned variable everything else in the first column refers to an assigned value  like     a   null   a        a   0       the columns refer to comparison operations  like     a     null isset  a  empty  a   a   true   false        All results are boolean  true is printed and false is omitted   You can run the tests yourself  check this gist  https   gist github com mfdj 8165967

User · Answer

Object properties can be checked for existence by property exists  Example from a unit test   function testPropertiesExist          sl   amp   this- gt system log       props   array  log id                       type                       message                       username                       ip address                       date added         foreach  props as  prop             this- gt assertTrue property exists  sl   prop                               Property  lt   prop  gt  exists

User · Answer

I prefer using not empty as the best method to check for the existence of a variable that a  exists  and b  is not null   if   empty  variable   do something

User · Answer

If the variable you are checking would be in the global scope you could do   array key exists  v    GLOBALS

User · Answer

If I run the following   echo   lt  php echo  foo    gt     php   I get an error   PHP Notice   Undefined variable  foo in  home altern8 - on line 1   If I run the following   echo   lt  php if   isset  foo      echo  foo      gt     php   I do not get the error    If I have a variable that should be set  I usually do something like the following    foo   isset  foo     foo   null    or  if     isset  foo     foo   null    That way  later in the script  I can safely use  foo and know that it  is set   and that it defaults to null  Later I can if   is null  foo                  if I need to and know for certain that the variable exists  even if it is null   The full isset documentation reads a little more than just what was initially pasted  Yes  it returns false for a variable that was previously set but is now null  but it also returns false if a variable has not yet been set  ever  and for any variable that has been marked as unset  It also notes that the NULL byte    0   is not considered null and will return true      Determine whether a variable is set       If a variable has been unset with   unset    it will no longer be set    isset   will return FALSE if testing a   variable that has been set to NULL    Also note that a NULL byte    0   is   not equivalent to the PHP NULL   constant

User · Answer

THE only way to know if a variable is defined in current scope   GLOBALS is not trustworthy  is array key exists   var name   get defined vars

User · Answer

Try using  unset  v    It seems the only time a variable is not set is when it is specifically unset  v   It sounds like your meaning of  existence  is different than PHP s definition  NULL is certainly existing  it is NULL

User · Answer

Explaining NULL  logically thinking  I guess the obvious answer to all of this is    Don t initialise your variables as NULL  initalise them as something relevant to what they are intended to become   Treat NULL properly  NULL should be treated as  non-existant value   which is the meaning of NULL  The variable can t be classed as existing to PHP because it hasn t been told what type of entity it is trying to be  It may aswell not exist  so PHP just says  Fine  it doesn t because there s no point to it anyway and NULL is my way of saying this    An argument  Let s argue now   But NULL is like saying 0 or FALSE or      Wrong  0-FALSE-   are all still classed as empty values  but they ARE specified as some type of value or pre-determined answer to a question  FALSE is the answer to yes or no    is the answer to the title someone submitted  and 0 is the answer to quantity or time etc  They ARE set as some type of answer result which makes them valid as being set   NULL is just no answer what so ever  it doesn t tell us yes or no and it doesn t tell us the time and it doesn t tell us a blank string got submitted  That s the basic logic in understanding NULL   Summary  It s not about creating wacky functions to get around the problem  it s just changing the way your brain looks at NULL  If it s NULL  assume it s not set as anything  If you are pre-defining variables then pre-define them as 0  FALSE or    depending on the type of use you intend for them   Feel free to quote this  It s off the top of my logical head

User · Answer

isset checks if the variable is set and  if so  whether its value is not NULL  The latter part is  in my opinion  not within the scope of this function  There is no decent workaround to determine whether a variable is NULL because it is not set or because it is explicitly set to NULL   Here is one possible solution    e1   error get last     isNULL   is null   x    e2   error get last     isNOTSET    e1     e2  echo sprintf  isNOTSET   d  isNULL   d    isNOTSET   isNULL       Sample output     when  x is not set  isNOTSET  1  isNULL  1    when  x   NULL      isNOTSET  0  isNULL  1    when  x   false     isNOTSET  0  isNULL  0   Other workaround is to probe the output of get defined vars      vars   get defined vars     isNOTSET    array key exists  x    vars    isNULL    isNOTSET   true   is null  x   echo sprintf  isNOTSET   d  isNULL   d    isNOTSET   isNULL       Sample output     when  x is not set  isNOTSET  1  isNULL  1    when  x   NULL      isNOTSET  0  isNULL  1    when  x   false     isNOTSET  0  isNULL  0

User · Answer

Sometimes I get a little lost trying to figure out which comparison operation to use in a given situation  isset   only applies to uninitialized or explicitly null values  Passing assigning null is a great way to ensure a logical comparison works as expected   Still  it s a little difficult to think about so here s a simple matrix comparing how different values will be evaluated by different operations                    null   is null   isset   empty   if else   ternary   count gt 0     -----       -----     -----     -----   -----   -----     -----     -----        a          true      true              true                                    null        true      true              true                                                                    true    true                                    0                               true    true                        true                                        true    true                        true        1                               true            true      true      true        -1                              true            true      true      true                                        true            true      true      true         str                            true            true      true      true         0 1                            true            true      true      true        new Class                       true            true      true      true        To fit the table I compressed the labels a bit     a  refers to a declared but unassigned variable everything else in the first column refers to an assigned value  like     a   null   a        a   0       the columns refer to comparison operations  like     a     null isset  a  empty  a   a   true   false        All results are boolean  true is printed and false is omitted   You can run the tests yourself  check this gist  https   gist github com mfdj 8165967

User · Answer

Attempting to give an overview of the various discussions and answers   There is no single answer to the question which can replace all the ways isset can be used  Some use cases are addressed by other functions  while others do not stand up to scrutiny  or have dubious value beyond code golf  Far from being  broken  or  inconsistent   other use cases demonstrate why isset s reaction to null is the logical behaviour   Real use cases  with solutions   1  Array keys  Arrays can be treated like collections of variables  with unset and isset treating them as though they were  However  since they can be iterated  counted  etc  a missing value is not the same as one whose value is null   The answer in this case  is to use array key exists   instead of isset     Since this is takes the array to check as a function argument  PHP will still raise  notices  if the array itself doesn t exist  In some cases  it can validly be argued that each dimension should have been initialised first  so the notice is doing its job  For other cases  a  recursive  array key exists function  which checked each dimension of the array in turn  would avoid this  but would basically be the same as  array key exists  It is also somewhat tangential to the handling of null values   2  Object properties  In the traditional theory of  Object-Oriented Programming   encapsulation and polymorphism are key properties of objects  in a class-based OOP implementation like PHP s  the encapsulated properties are declared as part of the class definition  and given access levels  public  protected  or private     However  PHP also allows you to dynamically add properties to an object  like you would keys to an array  and some people use class-less objects  technically  instances of the built in stdClass  which has no methods or private functionality  in a similar way to associative arrays  This leads to situations where a function may want to know if a particular property has been added to the object given to it   As with array keys  a solution for checking object properties is included in the language  called  reasonably enough  property exists   Non-justifiable use cases  with discussion  3  register globals  and other pollution of the global namespace  The register globals feature added variables to the global scope whose names were determined by aspects of the HTTP request  GET and POST parameters  and cookies   This can lead to buggy and insecure code  which is why it has been disabled by default since PHP 4 2  released Aug 2000 and removed completely in PHP 5 4  released Mar 2012  However  it s possible that some systems are still running with this feature enabled or emulated  It s also possible to  pollute  the global namespace in other ways  using the global keyword  or  GLOBALS array   Firstly  register globals itself is unlikely to unexpectedly produce a null variable  since the GET  POST  and cookie values will always be strings  with    still returning true from isset   and variables in the session should be entirely under the programmer s control   Secondly  pollution of a variable with the value null is only an issue if this over-writes some previous initialization   Over-writing  an uninitialized variable with null would only be problematic if code somewhere else was distinguishing between the two states  so on its own this possibility is an argument against making such a distinction   4  get defined vars and compact  A few rarely-used functions in PHP  such as get defined vars and compact  allow you to treat variable names as though they were keys in an array  For global variables  the super-global array  GLOBALS allows similar access  and is more common  These methods of access will behave differently if a variable is not defined in the relevant scope   Once you ve decided to treat a set of variables as an array using one of these mechanisms  you can do all the same operations on it as on any normal array  Consequently  see 1   Functionality that existed only to predict how these functions are about to behave  e g   will there be a key  foo  in the array returned by get defined vars    is superfluous  since you can simply run the function and find out with no ill effects   4a  Variable variables    foo   While not quite the same as functions which turn a set of variables into an associative array  most cases using  variable variables    assign to a variable named based on this other variable   can and should be changed to use an associative array instead    A variable name  fundamentally  is the label given to a value by the programmer  if you re determining it at run-time  it s not really a label but a key in some key-value store  More practically  by not using an array  you are losing the ability to count  iterate  etc  it can also become impossible to have a variable  outside  the key-value store  since it might be over-written by   foo   Once changed to use an associative array  the code will be amenable to solution 1  Indirect object property access  e g   foo- gt  property name  can be addressed with solution 2   5  isset is so much easier to type than array key exists  I m not sure this is really relevant  but yes  PHP s function names can be pretty long-winded and inconsistent sometimes  Apparently  pre-historic versions of PHP used a function name s length as a hash key  so Rasmus deliberately made up function names like htmlspecialchars so they would have an unusual number of characters     Still  at least we re not writing Java  eh      6  Uninitialized variables have a type  The manual page on variable basics includes this statement      Uninitialized variables have a default value of their type depending on the context in which they are used   I m not sure whether there is some notion in the Zend Engine of  uninitialized but known type  or whether this is reading too much into the statement   What is clear is that it makes no practical difference to their behaviour  since the behaviours described on that page for uninitialized variables are identical to the behaviour of a variable whose value is null  To pick one example  both  a and  b in this code will end up as the integer 42   unset  a    a    42    b   null   b    42     The first will raise a notice about an undeclared variable  in an attempt to make you write better code  but it won t make any difference to how the code actually runs    99  Detecting if a function has run   Keeping this one last  as it s much longer than the others  Maybe I ll edit it down later      Consider the following code    test value    hello   foreach    list of things as  thing         if   some test  thing   test value               result   some function  thing           if   isset  result          echo  The test passed at least once        If some function can return null  there s a possibility that the echo won t be reached even though some test returned true  The programmer s intention was to detect when  result had never been set  but PHP does not allow them to do so   However  there are other problems with this approach  which become clear if you add an outer loop   foreach    list of tests as  test value            something s missing here        foreach    list of things as  thing             if   some test  thing   test value                   result   some function  thing                       if   isset  result              echo  The test passed at least once              Because  result is never initialized explicitly  it will take on a value when the very first test passes  making it impossible to tell whether subsequent tests passed or not  This is actually an extremely common bug when variables aren t initialised properly   To fix this  we need to do something on the line where I ve commented that something s missing  The most obvious solution is to set  result to a  terminal value  that some function can never return  if this is null  then the rest of the code will work fine  If there is no natural candidate for a terminal value because some function has an extremely unpredictable return type  which is probably a bad sign in itself   then an additional boolean value  e g   found  could be used instead   Thought experiment one  the very null constant  PHP could theoretically provide a special constant - as well as null - for use as a terminal value here  presumably  it would be illegal to return this from a function  or it would be coerced to null  and the same would probably apply to passing it in as a function argument  That would make this very specific case slightly simpler  but as soon as you decided to re-factor the code - for instance  to put the inner loop into a separate function - it would become useless  If the constant could be passed between functions  you could not guarantee that some function would not return it  so it would no longer be useful as a universal terminal value   The argument for detecting uninitialised variables in this case boils down to the argument for that special constant  if you replace the comment with unset  result   and treat that differently from  result   null  you are introducing a  value  for  result that cannot be passed around  and can only be detected by specific built-in functions   Thought experiment two  assignment counter  Another way of thinking about what the last if is asking is  has anything made an assignment to  result   Rather than considering it to be a special value of  result  you could maybe think of this as  metadata  about the variable  a bit like Perl s  variable tainting   So rather than isset you might call it has been assigned to  and rather than unset  reset assignment state   But if so  why stop at a boolean  What if you want to know how many times the test passed  you could simply extend your metadata to an integer and have get assignment count and reset assignment count     Obviously  adding such a feature would have a trade-off in complexity and performance of the language  so it would need to be carefully weighed against its expected usefulness  As with a very null constant  it would be useful only in very narrow circumstances  and would be similarly resistant to re-factoring    The hopefully-obvious question is why the PHP runtime engine should assume in advance that you want to keep track of such things  rather than leaving you to do it explicitly  using normal code

User · Answer

I m going to add a quick two cents to this  One reason this issue is confusing is because this scenario seems to return the same result with error reporting not on full    a   null  var dump  a      NULL var dump  b      NULL   You could assume from this result that the difference between  a   null and not defining  b at all is nothing   Crank error reporting up   NULL  Notice  Undefined variable  b in xxx on line n NULL   Note  it threw an undefined variable error  but the output value of var dump is still NULL   PHP obviously does have an internal ability to distinguish between a null variable and an undefined variable  It seems to me that there should be a built in function to check for this   I think the accepted answer is good for the most part  but if I was going to implement it I would write a wrapper for it  As previously mentioned in this answer  I have to agree that I haven t actually encountered a situation where this has been a problem  I seem to almost always end up in a scenario where my variables are either set and defined  or they aren t  undefined  unset  null  blank  etc   Not to say that a situation like this won t occur in future  but as it seems to be quite a unique issue I m not surprised that the PHP devs haven t bothered to put this in

User · Answer

If the variable you are checking would be in the global scope you could do   array key exists  v    GLOBALS

User · Answer

I m going to add a quick two cents to this  One reason this issue is confusing is because this scenario seems to return the same result with error reporting not on full    a   null  var dump  a      NULL var dump  b      NULL   You could assume from this result that the difference between  a   null and not defining  b at all is nothing   Crank error reporting up   NULL  Notice  Undefined variable  b in xxx on line n NULL   Note  it threw an undefined variable error  but the output value of var dump is still NULL   PHP obviously does have an internal ability to distinguish between a null variable and an undefined variable  It seems to me that there should be a built in function to check for this   I think the accepted answer is good for the most part  but if I was going to implement it I would write a wrapper for it  As previously mentioned in this answer  I have to agree that I haven t actually encountered a situation where this has been a problem  I seem to almost always end up in a scenario where my variables are either set and defined  or they aren t  undefined  unset  null  blank  etc   Not to say that a situation like this won t occur in future  but as it seems to be quite a unique issue I m not surprised that the PHP devs haven t bothered to put this in

User · Answer

According to the PHP Manual for the empty   function   Determine whether a variable is considered to be empty  A variable is considered empty IF IT DOES NOT EXIST or if its value equals FALSE  empty   does not generate a warning if the variable does not exist     My emphasis    That means the empty   function should qualify as the  best way to test a variable s existence in PHP   per the title Question   However  this is not good enough  because the empty   function can be fooled by a variable that does exist and is set to NULL   I m interrupting my earlier answer to present something better  because it is less cumbersome than my original answer  which follows this interruption  for comparing      function undef  dnc    do not care what we receive      inf ob get contents                  get the content of the buffer     ob end clean                          stop buffering outputs  and empty the buffer     if  inf gt                               if test associated with the call to this function had an output       if false   strpos  inf   Undef      if the word  Undefined  was part of the output         return true                       tested variable is undefined           return false                          tested variable is not undefined       Two simple lines of code can use the above function to reveal if a variable is undefined     ob start                                pass all output messages  including errors  to a buffer   if undef  testvar   null                in this case the variable being tested is  testvar   You can follow those two lines with anything appropriate  such as this example       echo  variable is undefined      else     echo  variable exists  holding some value      I wanted to put the call to ob start   and the   testvar   null  inside the function  and simply pass the variable to the function  but it doesn t work   Even if you try to use  pass by reference  of the variable to the function  the variable BECOMES defined  and then the function can never detect that it previously had been undefined   What is presented here is a compromise between what I wanted to do  and what actually works   The preceding implies that there is another way to always avoid running into the  Undefined variable  error message    The assumption here is  preventing such a message is why you want to test to see if a variable is undefined       function inst  amp  v    return       receive any variable passed by reference  instantiates the undefined   Just call that function before doing something to your  testvar      inst  testvar                    The function doesn t affect any value of any already-existing variable   The newly-instantiated variable s value is set to null  of course    Interruption ends   So  after some studying and experimenting  here is something guaranteed to work    function myHndlr  en   es   ef   el     global  er      er    substr  es  0  18      Undefined variable       return        er   false   if empty  testvar      set error handler  myHndlr         testvar     null      restore error handler        if  er      will be 1  true  if the tested variable was not defined         do whatever you think is appropriate to the undefined variable      The explanation  A variable  er is initialized to a default value of  no error    A  handler function  is defined   If the  testvar  the variable we want to know whether or not is undefined  passes the preliminary empty   function test  then we do the more thorough test   We call the set error handler   function to use the previously-defined handler function   Then we do a simple identity-comparison involving  testvar  WHICH IF UNDEFINED WILL TRIGGER AN ERROR   The handler function captures the error and specifically tests to see if the reason for the error is the fact that the variable is undefined   The result is placed in the error-information variable  er  which we can later test to do whatever we want as a result of knowing for sure whether or not  testvar was defined   Because we only need the handler function for this limited purpose  we restore the original error-handling function   The  myHndlr  function only needs to be declared once  the other code can be copied to whatever places are appropriate  for  testvar or any other variable we want to test this way

User · Answer

I have to say in all my years of PHP programming, I have never encountered a problem with isset() returning false on a null variable. OTOH, I have encountered problems with isset() failing on a null array entry - but array_key_exists() works correctly in that case.

For some comparison, Icon explicitly defines an unused variable as returning &null so you use the is-null test in Icon to also check for an unset variable. This does make things easier. On the other hand, Visual BASIC has multiple states for a variable that doesn't have a value (Null, Empty, Nothing, ...), and you often have to check for more than one of them. This is known to be a source of bugs.

User · Answer

As an addition to greatbigmassive s discussion of what NULL means  consider what  the existence of a variable  actually means   In many languages  you have to explicitly declare every variable before you use it  this may determine its type  but more importantly it declares its scope  A variable  exists  everywhere in its scope  and nowhere outside it - be that a whole function  or a single  block    Within its scope  a variable assigns some meaning to a label which you  the programmer  have chosen  Outside its scope  that label is meaningless  whether you use the same label in a different scope is basically irrelevant    In PHP  variables do not need to be declared - they come to life as soon as you need them  When you write to a variable for the first time  PHP allocates an entry in memory for that variable  If you read from a variable that doesn t currently have an entry  PHP considers that variable to have the value NULL   However  automatic code quality detectors will generally warn you if you use a variable without  initialising  it first  Firstly  this helps detect typos  such as assigning to  thingId but reading from  thing id  but secondly  it forces you to consider the scope over which that variable has meaning  just as a declaration would   Any code that cares whether a variable  exists  is part of the scope of that variable - whether or not it has been initialised  you as a programmer have given that label meaning at that point of the code  Since you re using it  it must in some sense  exist   and if it exists  it must have an implicit value  in PHP  that implicit value is null   Because of the way PHP works  it is possible to write code that treats the namespace of existent variables not as a scope of labels you have given meaning to  but as some kind of key-value store  You can  for instance  run code like this   var     GET  var name      var     GET  var value     Just because you can  doesn t mean it s a good idea   It turns out  PHP has a much better way of representing key-value stores  called associative arrays  And although the values of an array can be treated like variables  you can also perform operations on the array as a whole  If you have an associative array  you can test if it contains a key using array key exists      You can also use objects in a similar way  dynamically setting properties  in which case you can use property exists   in exactly the same way  Of course  if you define a class  you can declare which properties it has - you can even choose between public  private  and protected scope    Although there is a technical difference between a variable  as opposed to an array key  or an object property  that hasn t been initialised  or that has been explicitly unset    and one whose value is null  any code that considers that difference to be meaningful is using variables in a way they re not meant to be used

User · Answer

I have to say in all my years of PHP programming, I have never encountered a problem with isset() returning false on a null variable. OTOH, I have encountered problems with isset() failing on a null array entry - but array_key_exists() works correctly in that case.

For some comparison, Icon explicitly defines an unused variable as returning &null so you use the is-null test in Icon to also check for an unset variable. This does make things easier. On the other hand, Visual BASIC has multiple states for a variable that doesn't have a value (Null, Empty, Nothing, ...), and you often have to check for more than one of them. This is known to be a source of bugs.

User · Answer

I don t agree with your reasoning about NULL  and saying that you need to change your mindset about NULL is just weird   I think isset   was not designed correctly  isset   should tell you if the variable has been set and it should not be concerned with the actual value of the variable   What if you are checking values returned from a database and one of the columns have a NULL value  you still want to know if it exists even if the value is NULL   nope dont trust isset   here   likewise    a   array   test    gt  1   hello    gt  NULL    var dump isset  a  test           TRUE var dump isset  a  foo            FALSE var dump isset  a  hello          FALSE   isset   should have been designed to work like this   if isset  var   amp  amp   var   NULL         this way we leave it up to the programmer to check types and not leave it up to isset   to assume its not there because the value is NULL - its just stupid design

User · Answer

If I run the following   echo   lt  php echo  foo    gt     php   I get an error   PHP Notice   Undefined variable  foo in  home altern8 - on line 1   If I run the following   echo   lt  php if   isset  foo      echo  foo      gt     php   I do not get the error    If I have a variable that should be set  I usually do something like the following    foo   isset  foo     foo   null    or  if     isset  foo     foo   null    That way  later in the script  I can safely use  foo and know that it  is set   and that it defaults to null  Later I can if   is null  foo                  if I need to and know for certain that the variable exists  even if it is null   The full isset documentation reads a little more than just what was initially pasted  Yes  it returns false for a variable that was previously set but is now null  but it also returns false if a variable has not yet been set  ever  and for any variable that has been marked as unset  It also notes that the NULL byte    0   is not considered null and will return true      Determine whether a variable is set       If a variable has been unset with   unset    it will no longer be set    isset   will return FALSE if testing a   variable that has been set to NULL    Also note that a NULL byte    0   is   not equivalent to the PHP NULL   constant

User · Answer

I think the only full solution is to report notices with  error reporting E ALL      Enables E NOTICE   But you will have to fix all the notices generated by undefined variables  constants  array keys  class properties amongst others  Once you have done that you won t have to worry about the difference between null and not declared variables  and the ambiguity dissappears   Enabling notice reporting might not be a good alternative in all situations  but there are good reasons to enable it   Why should I fix E NOTICE errors   In my case was more than a year working in a proyect without it  but was used to be careful about declaring variables  so it was fast to transition

User · Answer

Try using  unset  v    It seems the only time a variable is not set is when it is specifically unset  v   It sounds like your meaning of  existence  is different than PHP s definition  NULL is certainly existing  it is NULL

[php] Best way to test for a variable's existence in PHP; isset() is clearly broken

Examples related to php

Examples related to variables

Examples related to isset