Null vs False vs 0 in PHP

Question

I am told that good developers can spot utilize the difference between Null and False and 0 and all the other good  nothing  entities  What is the difference  specifically in PHP   Does it have something to do with

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In pretty much all modern languages  null logically refers to pointers  or references  not having a value  or a variable that is not initialized   0 is the integer value of zero  and false is the boolean value of  well  false   To make things complicated  in C  for example  null  0  and false are all represented the exact same way   I don t know how it works in PHP   Then  to complicate things more  databases have a concept of null  which means missing or not applicable  and most languages don t have a direct way to map a DBNull to their null   Until recently  for example  there was no distinction between an int being null and being zero  but that was changed with nullable ints   Sorry to make this sound complicated   It s just that this has been a harry sticking point in languages for years  and up until recently  it hasn t had any clear resolution anywhere   People used to just kludge things together or make blank or 0 represent nulls in the database  which doesn t always work too well

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False and 0 are conceptually similar  i e  they are isomorphic  0 is the initial value for the algebra of natural numbers  and False is the initial value for the Boolean algebra   In other words  0 can be defined as the number which  when added to some natural number  yields that same number   x   0   x   Similarly  False is a value such that a disjunction of it and any other value is that same value   x    False   x   Null is conceptually something totally different  Depending on the language  there are different semantics for it  but none of them describe an  initial value  as False and 0 are  There is no algebra for Null  It pertains to variables  usually to denote that the variable has no specific value in the current context  In most languages  there are no operations defined on Null  and it s an error to use Null as an operand  In some languages  there is a special value called  bottom  rather than  null   which is a placeholder for the value of a computation that does not terminate   I ve written more extensively about the implications of NULL elsewhere

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It s language specific  but in PHP    Null means  nothing   The var has not been initialized   False means  not true in a boolean context   Used to explicitly show you are dealing with logical issues   0 is an int  Nothing to do with the rest above  used for mathematics   Now  what is tricky  it s that in dynamic languages like PHP  all of them have a value in a boolean context  which  in PHP  is False   If you test it with     it s testing the boolean value  so you will get equality  If you test it with      it will test the type  and you will get inequality   So why are they useful    Well  look at the strrpos   function  It returns False if it did not found anything  but 0 if it has found something at the beginning of the string     lt  php    pitfall   if  strrpos  Hello World    Hello              never exectuted       smart move   if  strrpos  Hello World    Hello       False           that works       gt    And of course  if you deal with states   You want to make a difference between DebugMode   False  set to off   DebugMode   True  set to on  and DebugMode   Null  not set at all  will lead to hard debugging  -

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From the PHP online documentation      To explicitly convert a value to boolean  use the  bool  or  boolean  casts    However  in most cases the cast is unncecessary  since a value will be automatically converted if an operator  function or control structure requires a boolean argument    When converting to boolean  the following values are considered FALSE       the boolean FALSE itself   the integer   0  zero    the float 0 0  zero    the empty string  and the string  0    an array with zero elements   an object with zero member variables  PHP 4 only    the special type NULL  including unset variables    SimpleXML objects created from empty tags Every other value is considered TRUE  including any resource         So  in most cases  it s the same     On the other hand  the     and the   are not the same thing  Regularly  you just need the  equals  operator  To clarify      a     b      Equal  TRUE if  a is equal to  b   a      b     Identical  TRUE if  a is equal to  b  and they are of the same type     For more information  check the  Comparison Operators  page in the PHP online docs     Hope this helps

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Well  I can t remember enough from my PHP days to answer the       part  but for most C-style languages  NULL should be used in the context of pointer values  false as a boolean  and zero as a numeric value such as an int     0  is the customary value for a character context   I usually also prefer to use 0 0 for floats and doubles   So   the quick answer is  context

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False  Null  Nothing  0  Undefined  etc   etc   Each of these has specific meanings that correlate with actual concepts  Sometimes multiple meanings are overloaded into a single keyword or value   In C and C    NULL  False and 0 are overloaded to the same value  In C  they re 3 distinct concepts   null or NULL usually indicates a lack of value  but usually doesn t specify why  0 indicates the natural number zero and has type-equivalence to 1  2  3  etc  and in languages that support separate concepts of NULL should be treated only a number   False indicates non-truth  And it used in binary values  It doesn t mean unset  nor does it mean 0  It simply indicates one of two binary values   Nothing can indicate that the value is specifically set to be nothing which indicates the same thing as null  but with intent   Undefined in some languages indicates that the value has yet to be set because no code has specified an actual value

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In PHP you can use     and     operators to check not only if the values are equal but also if their types match  So for example  0    false is true  but 0     false is false  The same goes for    versus      Also in case you compare null to the other two using the mentioned operators  expect similar results   Now in PHP this quality of values is usually used when returning a value which sometimes can be 0  zero   but sometimes it might be that the function failed  In such cases in PHP you return false and you have to check for these cases using the identity operator      For example if you are searching for a position of one string inside the other and you re using strpos    this function will return the numeric position which can be 0 if the string is found at the very beginning  but if the string is not found at all  then strpos   will return false and you have to take this into account when dealing with the result   If you will use the same technique in your functions  anybody familiar with the standard PHP library will understand what is going on and how to check if the returned value is what is wanted or did some error occur while processing  The same actually goes for function params  you can process them differently depending on if they are arrays or strings or what not  and this technique is used throughout PHP heavily too  so everybody will get it quite easily  So I guess that s the power

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Null is nothing  False is a bit  and 0 is  probably  32 bits   Not a PHP expert  but in some of the more modern languages those aren t interchangeable   I kind of miss having 0 and false be interchangeable  but with boolean being an actual type you can have methods and objects associated with it so that s just a tradeoff   Null is null though  the absence of anything essentially

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Somebody can explain to me why  NULL  is not just a string in a comparison instance    x   0  var dump  x     NULL       TRUE      WTF

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Below is an example               Comparisons of  x with PHP functions  Expression          gettype     empty       is null     isset   boolean   if  x   x                  string      TRUE        FALSE       TRUE    FALSE  x   null           NULL        TRUE        TRUE        FALSE   FALSE var  x              NULL        TRUE        TRUE        FALSE   FALSE  x is undefined     NULL        TRUE        TRUE        FALSE   FALSE  x   array          array       TRUE        FALSE       TRUE    FALSE  x   false          boolean     TRUE        FALSE       TRUE    FALSE  x   true           boolean     FALSE       FALSE       TRUE    TRUE  x   1              integer     FALSE       FALSE       TRUE    TRUE  x   42             integer     FALSE       FALSE       TRUE    TRUE  x   0              integer     TRUE        FALSE       TRUE    FALSE  x   -1             integer     FALSE       FALSE       TRUE    TRUE  x    1             string      FALSE       FALSE       TRUE    TRUE  x    0             string      TRUE        FALSE       TRUE    FALSE  x    -1            string      FALSE       FALSE       TRUE    TRUE  x    php           string      FALSE       FALSE       TRUE    TRUE  x    true          string      FALSE       FALSE       TRUE    TRUE  x    false         string      FALSE       FALSE       TRUE    TRUE   Please see this for more reference of type comparisons in PHP  It should give you a clear understanding

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Null is used in databases to represent  no record  or  no information   So you might have a bit field that describes  does this user want to be sent e-mails by us   where True means they do  False means they don t want to be sent anything  but Null would mean that you don t know  They can come about through outer joins and suchlike    The logical implications of Null are often different - in some languages NULL is not equal to anything  so if a    NULL  will always be false   So personally I d always initialise a boolean to FALSE  and initialising one to NULL would look a bit icky  even in C where the two are both just 0    just a style thing

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I have just wasted 1 2 a day trying to get either a 0  null  false to return from strops   Here s all I was trying to do  before I found that the logic wasn t flowing in the right direction  seeming that there was a blackhole in php coding   Concept take a domain name hosted on a server  and make sure it s not root level  OK several different ways to do this  but I chose different due to other php functions  constructs I have done   Anyway here was the basis of the cosing   if  strpos   SERVER   SERVER NAME    dirBaseNAME             do this    else       or that      echo strpos mydomain co uk  mydomain      if   strpos mydomain  xmas     null                  echo   n1 is null           if    strpos mydomain co uk  mydomain      0                  echo   n2 is 0          else           echo   n2 Something is WRONG           if    mydomain co uk  mydomain      0                  echo   n3 is 0          else           echo   n3 it is not 0           if    mydomain co uk  mydomain      null                  echo   n4 is null          else           echo   n4 Something is WRONG              FINALLY after reading this Topic   I found that this worked       if   mydomain co uk  mydomain       false                  echo   n5 is True          else           echo   n5 is False              Thanks for this article  I now understand that even though it s Christmas  it may not be Christmas as false  as its also can be a NULL day   After wasting a day of debugging some simple code  wished I had known this before  as I would have been able to identify the problem  rather than going all over the place trying to get it to work  It didn t work  as False  NULL and 0 are not all the same as True or False or NULL

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null is null  false is false  Sad but true  there s not much consistency in PHP  though it is improving on latest releases  there s too much backward compatibility   Despite the design wishing some consistency  outlined in the selected answer here   it all get confusing when you consider method returns that use false null in not-so-easy to reason ways  You will often see null being used when they are already using false for something  e g  filter input    They return false if the variable fails the filter  and null if the variable does not exists  does not existing means it also failed the filter   Methods returning false null string etc interchangeably is a hack when the author care about the type of failure  for example  with filter input   you can check for    false or    null if you care why the validation failed  But if you don t it might be a pitfall as one might forget to add the check for    null if they only remembered to write the test case for    false  And most php unit test coverage tools will not call your attention for the missing  untested code path  Lastly  here s some fun with type juggling  not even including arrays or objects  var dump  0 lt 0            bool false  var dump  1 lt 0            bool false  var dump  -1 lt 0           bool true  var dump  false lt 0        bool false  var dump  null lt 0         bool false  var dump     lt 0           bool false  var dump   a  lt 0          bool false  echo  quot  n quot   var dump   0            bool true  var dump   1            bool false  var dump   -1           bool false  var dump   false        bool true  var dump   null         bool true  var dump                bool true  var dump    a           bool false  echo  quot  n quot   var dump  false    0            bool true  var dump  false    1            bool false  var dump  false    -1           bool false  var dump  false    false        bool true  var dump  false    null         bool true  var dump  false                 bool true  var dump  false     a           bool false  echo  quot  n quot   var dump  null    0            bool true  var dump  null    1            bool false  var dump  null    -1           bool false  var dump  null    false        bool true  var dump  null    null         bool true  var dump  null                 bool true  var dump  null     a           bool false  echo  quot  n quot    a 0  var dump  empty  a             bool true   a 1  var dump  empty  a             bool false   a -1  var dump  empty  a            bool false   a false  var dump  empty  a         bool true   a null  var dump  empty  a          bool true   a     var dump  empty  a            bool true   a  a   var dump  empty  a           bool false  echo  quot  n quot    new block suggested by  thehpi var dump  null  lt  -1     bool true  var dump  null  lt  0     bool false  var dump  null  lt  1     bool true  var dump  -1  gt  true     bool false  var dump  0  gt  true     bool false  var dump  1  gt  true     bool true  var dump  -1  gt  false     bool true  var dump  0  gt  false     bool false  var dump  1  gt  true     bool true

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In PHP it depends on if you are validating types         false     0    amp  amp    false     -1    amp  amp    false    0    amp  amp    false    -1    amp  amp     false     null    amp  amp    false    null        Technically null is 0x00 but in PHP   null    0x00    amp  amp    null     0x00     0 is an integer value

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The issues with falsyness comes from the PHP history  The problem targets the not well defined scalar type          true - gt  true  string match          true - gt  false  numberic match    int        true - gt  false  string        true - gt  true   PHP7 strictness is a step forward  but maybe not enough  https   web-techno net typing-with-php-7-what-you-shouldnt-do

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One interesting fact about NULL in PHP  If you set a var equal to NULL  it is the same as if you had called unset   on it   NULL essentially means a variable has no value assigned to it  false is a valid Boolean value  0 is a valid integer value  and PHP has some fairly ugly conversions between 0   0       and false

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I think bad developers find all different uses of null 0 false in there code   For example  one of the most common mistakes developers make is to return error code in the form of data with a function      On error GetChar returns -1 int GetChar     This is an example of a sugar interface  This is exsplained in the book  Debuging the software development proccess  and also in another book  writing correct code     The problem with this  is the implication or assumptions made on the char type  On some compilers the char type can be non-signed  So even though you return a -1 the compiler can return 1 instead  These kind of compiler assumptions in C   or C are hard to spot   Instead  the best way is not to mix error code with your data  So the following function   char GetChar     now becomes     On success return 1    on failure return 0 bool GetChar int  amp char    This means no matter how young the developer is in your development shop  he or she will never get this wrong  Though this is not talking about redudancy or dependies in code   So in general  swapping bool as the first class type in the language is okay and i think joel spoke about it with his recent postcast  But try not to use mix and match bools with your data in your routines and you should be perfectly fine

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The differences between these values always come down to detailed language-specific rules  What you learn for PHP isn t necessarily true for Python  or Perl  or C  etc  While it is valuable to learn the rules for the language s  you re working with  relying on them too much is asking for trouble  The trouble comes when the next programmer needs to maintain your code and you ve used some construct that takes advantage of some little detail of Null vs  False  for example   Your code should look correct  and conversely  wrong code should look wrong

[php] Null vs. False vs. 0 in PHP

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