International PHP Conference 2015

NULL

The special NULL value represents a variable with no value. NULL is the only possible value of type null.

A variable is considered to be null if:

  • it has been assigned the constant NULL.

  • it has not been set to any value yet.

  • it has been unset().

Syntax

There is only one value of type null, and that is the case-insensitive constant NULL.

<?php
$var 
NULL;       
?>

See also the functions is_null() and unset().

Casting to NULL

Casting a variable to null using (unset) $var will not remove the variable or unset its value. It will only return a NULL value.

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User Contributed Notes 7 notes

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32
quickpick
4 years ago
Note: empty array is converted to null by non-strict equal '==' comparison. Use is_null() or '===' if there is possible of getting empty array.

$a = array();

$a == null  <== return true
$a === null < == return false
is_null($a) <== return false
up
7
kuzawinski dot marcin at NOSPAM dot gmail dot com
1 year ago
Funny. It looks like, that there is one, and only one possible value for variable $a that will pass this test:

($a != NULL) && ((bool)$a == NULL)

It's "0" and it works because casting string "0" to boolean gives FALSE (and it's the only non empty string, that works this way). So remember that casting is not "transitive".
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9
nl-x at bita dot nl
8 years ago
Watch out. You can define a new constant with the name NULL with define("NULL","FOO");. But you must use the function constant("NULL"); to get it's value. NULL without the function call to the constant() function will still retrieve the special type NULL value.
Within a class there is no problem, as const NULL="Foo"; will be accessible as myClass::NULL.
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1
Toycat
2 years ago
Be careful using NULL together with namespaces. If a NULL constant is redefined in a namespace other than global, you will get unexpected results when comparing to NULL inside the namespace. Instead always use \NULL, \FALSE, and \TRUE when comparing. Otherwise it may lead to application failures and potential security issues where certain checks could be effectively disabled.

A simple example to demonstrate the behavior:

<?php
namespace RedefinedConstants {

   
// redefining global namespace constants has no effect
   
define('NULL', 'I am not global NULL!');
   
define('TRUE', 'I am not global TRUE!');
   
define('FALSE', 'I am not global FALSE!');

   
// redefining local namespace constants will work
   
define('RedefinedConstants\NULL', 'I am not NULL!', \TRUE);
   
define('RedefinedConstants\FALSE', 'I am not FALSE!', \TRUE);
   
define('RedefinedConstants\TRUE', 'I am not TRUE!', \TRUE);

   
var_dump(
       
NULL, \NULL, null, \null, Null, \Null,
       
FALSE, \FALSE, false, \false, False, \False,
       
TRUE, \TRUE, true, \true, True, \True
   
);

}
?>
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-8
foxdie_cs at hotmail dot com
3 years ago
a quick note about the magic function __get() :

<?php
class Foo{
   
    protected
$bar;
   
    public function
__construct(){
       
       
$this->bar = NULL;
       
var_dump( $this->bar ); //prit 'NULL' but won't call the magic method __get()
       
       
unset( $this->bar );
       
var_dump( $this->bar ); //print 'GET bar' and 'NULL'
           
   
}
   
    public function
__get( $var ){ echo "GET " . $var; }
       
}

new
Foo();
?>
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-8
dward at maidencreek dot com
13 years ago
Nulls are almost the same as unset variables and it is hard to tell the difference without creating errors from the interpreter:

<?php
$var
= NULL;
?>

isset($var) is FALSE
empty($var) is TRUE
is_null($var) is TRUE

isset($novar) is FALSE
empty($novar) is TRUE
is_null($novar) gives an Undefined variable error

$var IS in the symbol table (from get_defined_vars())
$var CAN be used as an argument or an expression.

So, in most cases I found that we needed to use !isset($var) intead of is_null($var) and then set $var = NULL if the variable needs to be used later to guarantee that $var is a valid variable with a NULL value instead of being undefined.
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-26
Anonymous
9 years ago
// Difference between "unset($a);" and "$a = NULL;" :
<?php
// unset($a)
$a = 5;
$b = & $a;
unset(
$a);
print
"b $b "; // b 5

// $a = NULL; (better I think)
$a = 5;
$b = & $a;
$a = NULL;
print
"b $b "; // b
print(! isset($b)); // 1
?>
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