php教程

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。

  • 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()unset()

转换到 NULL

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

User Contributed Notes

Anonymous 11-Jan-2018 07:38
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
Hayley Watson 07-Dec-2017 11:07
NULL is supposed to indicate the absence of a value, rather than being thought of as a value itself. It's the empty slot, it's the missing information, it's the unanswered question. It's not a jumped-up zero or empty set.

This is why a variable containing a NULL is considered to be unset: it doesn't have a value. Setting a variable to NULL is telling it to forget its value without providing a replacement value to remember instead. The variable remains so that you can give it a proper value to remember later; this is especially important when the variable is an array element or object property.

It's a bit of semantic awkwardness to speak of a "null value", but if a variable can exist without having a value, the language and implementation have to have something to represent that situation. Because someone will ask. If only to see if the slot has been filled.
kuzawinski dot marcin at NOSPAM dot gmail dot com 26-Mar-2014 12:10
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".
quickpick 22-Apr-2011 03:36
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 10:33
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.

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