PHP - Manual: NULL

2024-11-23

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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。
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`

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 将不会删除该变量或 unset 其值。仅是返回 null 值而已。

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User Contributed Notes 6 notes

down

quickpick ¶

11 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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mattias at kregert dot se ¶

10 months 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.

down

Hayley Watson ¶

4 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.

down

Anonymous ¶

4 years ago


Note: Non Strict Comparison '==' returns bool(true) for 

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false

down

hydrogen at live dot in ¶

1 year 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 :)

down

-10

Mojo ¶

1 year 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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官方地址：https://www.php.net/manual/en/language.types.null.php

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PHP官方手册 - 互联网笔记

NULL

语法

转换到 `NULL`

User Contributed Notes 6 notes

略微加速

PHP官方手册 - 互联网笔记

NULL

语法

转换到 NULL

User Contributed Notes 6 notes

转换到 `NULL`