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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

警告

本特性已自 PHP 7.2.0 起废弃。强烈建议不要使用本特性。

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。

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User Contributed Notes 5 notes

up
91
quickpick
10 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
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18
Hayley Watson
3 years ago
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.
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14
Anonymous
3 years ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
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0
hydrogen at live dot in
1 month ago
I would like to add for clarification that:

$x=NULL;

--$x;
// $x is still NULL.
// Decrementing NULL, using Decrement Operator, gives NULL.

$x-=1;
// $x is now int(-1).
// This actually decrements value by 1.

On the other hand, Incrementation works simply as expected.
Hope this helps :)
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-5
Mojo
3 months ago
Pay attention then using operator -- on NULL values:

$x = null;
--$x;      // $x is NULL
$x--;      // still NULL
$x -= 1;   // $x is -1

On other side for ++ everything works fine:

$x = null;
++$x;      // $ix is 1
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