NULL

特殊的 NULL 值表示一个变量没有值。NULL 类型唯一可能的值就是 NULL。

在下列情况下一个变量被认为是 NULL：

被赋值为 NULL。
尚未被赋值。
被 unset()。

语法

NULL 类型只有一个值，就是不区分大小写的常量 NULL。


<?php
$var = NULL;       
?>

参见 is_null() 和 unset()。

转换到 NULL

使用 (unset) $var 将一个变量转换为 null 将不会删除该变量或 unset 其值。仅是返回 NULL 值而已。

User Contributed Notes

sebastian dot goendoer at NOSPAM dot telekom dot de 03-Dec-2015 12:39


Valiable values of (almost) all types that can be juggled to something "like" NULL will eval to NULL:



<?php

$test['_string'] = "";

$test['_int'] = 0;

$test['_float'] = 0.0;

$test['_null'] = null;

$test['_arr'] = array();

$test['_obj'] = new \stdClass();



foreach($test as $key => $value)

{

    echo $key.": ";

    if($value == null)

        echo "is null\n";

    else

        echo "is not null\n";

}die();

?>



will give you



_string: is null

_int: is null

_float: is null

_null: is null

_arr: is null

_obj: is not null

kuzawinski dot marcin at NOSPAM dot gmail dot com 26-Mar-2014 09:10


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".

Toycat 29-May-2013 06:11


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


    );





}


?>

foxdie_cs at hotmail dot com 01-Aug-2012 03:00


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();


?>

quickpick 23-Apr-2011 12:36


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

nl-x at bita dot nl 09-Jul-2007 07:33


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.

06-Jan-2006 10:51


// 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 

?>

dward at maidencreek dot com 13-Nov-2001 12:52


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.