PHP 8.3.0 RC 6 available for testing

NULL

null 类型是 PHP 的原子类型(unit type),也就是说,它仅有一个值 null

未定义和 unset() 的变量都将解析为值 null

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;
?>

转换到 null

警告

本特性自 PHP 7.2.0 起废弃,并且自 PHP 8.0.0 起被移除。 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 6 notes

up
85
quickpick
12 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
42
Hayley Watson
5 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.
up
9
hydrogen at live dot in
2 years 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 :)
up
14
Anonymous
5 years ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
up
-5
mattias at kregert dot se
2 years ago
Note that NULL works like a magic object with any attribute you can name, but they are all NULL:

foreach ( [ null, null ] as $person ) {
$friends[] = [ 'Name'=>$person['name'], 'Phone'=>$person['cell'] ];
}

print_r($friends);

Array
(
[0] => Array
(
[Name] =>
[Phone] =>
)

[1] => Array
(
[Name] =>
[Phone] =>
)

)

This means that:
* NULL == NULL['foo']['bar']['whatever']

This can be slightly confusing if you accidentally slip a NULL into an array of other items.
up
-19
Mojo
2 years 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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