NULL

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文字

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。

用户评论:

[#1] kuzawinski dot marcin at NOSPAM dot gmail dot com [2014-03-26 00:10:50]

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".

[#2] Toycat [2013-05-29 09:11:09]

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 ); } ?>

[#3] foxdie_cs at hotmail dot com [2012-08-01 06:00:39]

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(); ?>

[#4] quickpick [2011-04-22 15:36:37]

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

[#5] nl-x at bita dot nl [2007-07-09 10:33:46]

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.

[#6] [2006-01-06 01:51:13]

// 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 ?>

[#7] dward at maidencreek dot com [2001-11-12 15:52:06]

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.