PHP - check if variable is undefined

Question

Consider this Javascript statement isTouch   document createTouch     undefined  I would like to know if we have a similar statement in PHP  not being isset   but literally checking for an undefined value  something like   isTouch     quot  quot  Is there something similar as the above in PHP

User · Answer

You can use the PHP isset   function to test whether a variable is set or not  The isset   will return FALSE if testing a variable that has been set to NULL  Example   lt  php      var1           if isset  var1            echo  This line is printed  because the  var1 is set            gt   This code will output  quot This line is printed  because the  var1 is set  quot  read more in https   stackhowto com how-to-check-if-a-variable-is-undefined-in-php

User · Answer

You can use -  Ternary oprator to check wheather value set by POST GET or not somthing like this   value1     POST  value1     isset   POST  value1        POST  value1          value2     POST  value2     isset   POST  value2        POST  value2          value3     POST  value3     isset   POST  value3        POST  value3          value4     POST  value4     isset   POST  value4        POST  value4

User · Answer

JavaScript s  strict not equal  operator       on comparison with undefined does not result in false on null values   var createTouch   null  isTouch   createTouch     undefined     true   To achieve an equivalent behaviour in PHP  you can check whether the variable name exists in the keys of the result of get defined vars        just to simplify output format const BR     lt br gt     PHP EOL      set a global variable to test independence in local scope  test   1      test in local scope  what is working in global scope as well  function test          is global variable found    echo   test       array key exists  test   get defined vars                           exists      does not exist       BR        test does not exist        is local variable found     test   null    echo   test       array key exists  test   get defined vars                           exists      does not exist       BR        test exists        try same non-null variable value as globally defined as well    test   1    echo   test       array key exists  test   get defined vars                           exists      does not exist       BR        test exists        repeat test after variable is unset   unset  test     echo   test       array key exists  test   get defined vars                           exists      does not exist      BR        test does not exist     test      In most cases  isset  variable  is appropriate  That is aquivalent to array key exists  variable   get defined vars     amp  amp  null      variable  If you just use null      variable without prechecking for existence  you will mess up your logs with warnings because that is an attempt to read the value of an undefined variable   However  you can apply an undefined variable to a reference without any warning      write our own isset   function function my isset  amp  var         here  var is defined      and initialized to null if the given argument was not defined   return null      var        passing an undefined variable by reference does not log any warning  is set   my isset  undefined variable         is set is false

User · Answer

An another way is simply    if  test       echo  Yes 1     if  is null  test        echo  Yes 2       test    hello    if  test       echo  Yes 3       Will return     Yes 3    The best way is to use isset    otherwise you can have an error like  undefined  test    You can do it like this    if  isset  test   amp  amp    test   null      You ll not have any error because the first condition isn t accepted

User · Answer

if isset  variable         isTouch    variable      OR  if  isset  variable         isTouch

User · Answer

You can use -    isTouch   isset  variable     It will return true if the  variable is defined   if the variable is not defined it will return false       Note   Returns TRUE if var exists and has value other than NULL  FALSE otherwise    If you want to check for false  0 etc You can then use empty   -    isTouch   empty  variable     empty   works for -        an empty string  0  0 as an integer  0 0  0 as a float   0   0 as a string  NULL FALSE array    an empty array   var   a variable declared  but without a value

User · Answer

To check is variable is set you need to use isset function    lorem    potato    if isset  lorem        echo  isset true      lt br   gt     else      echo  isset false      lt br   gt       if isset  ipsum        echo  isset true      lt br   gt     else      echo  isset false      lt br   gt        this code will print   isset true isset false   read more in https   php net manual en function isset php

User · Answer

The isset   function does not check if a variable is defined  It seems you ve specifically stated that you re not looking for isset   in the question  I don t know why there are so many answers stating that isset   is the way to go  or why the accepted answer states that as well  It s important to realize in programming that null is something  I don t know why it was decided that isset   would return false if the value is null  To check if a variable is undefined you will have to check if the variable is in the list of defined variables  using get defined vars    There is no equivalent to JavaScript s undefined  which is what was shown in the question  no jQuery being used there   In the following example it will work the same way as JavaScript s undefined check   isset   isset  variable   var dump  isset      false  But in this example  it won t work like JavaScript s undefined check   variable   null   isset   isset  variable   var dump  isset      false   variable is being defined as null  but the isset   call still fails  So how do you actually check if a variable is defined  You check the defined variables  Using get defined vars   will return an associative array with keys as variable names and values as the variable values  We still can t use isset get defined vars    variable    here because the key could exist and the value still be null  so we have to use array key exists  variable   get defined vars      variable   null   isset   array key exists  variable   get defined vars     var dump  isset      true    isset   array key exists  otherVariable   get defined vars     var dump  isset      false  However  if you re finding that in your code you have to check for whether a variable has been defined or not  then you re likely doing something wrong  This is my personal belief as to why the core PHP developers left isset   to return false when something is null

[php] PHP - check if variable is undefined

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