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`

Warning

This feature has been DEPRECATED as of PHP 7.2.0, and REMOVED as of PHP 8.0.0. Relying on this feature is highly discouraged.

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

mattias at kregert dot se 22-Jul-2021 07:57


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.

hydrogen at live dot in 10-Apr-2021 03:39


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 :)

Mojo 10-Feb-2021 10:18


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

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.

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

NULL

语法

转换到 NULL

User Contributed Notes

转换到 `NULL`