NULL

null 型は、PHP では意味のある情報を持たないことを示す型(unit 型)です。 つまり、単一の値 null しか持ちません。

未定義、そして unset() された値は、値 null に解決されます。

文法

null 型の値は一つだけで、 大文字小文字を区別しない定数 null です。

<?php
$var 
NULL;       
?>

null へのキャスト

警告

この機能は PHP 7.2.0 で 非推奨 になり、PHP 8.0.0 で 削除 されました。この機能に頼らないことを強く推奨します。

(unset) $var を使って変数を null にキャストすると、その変数を削除したり値の設定を解除したりは しません。単に null 値を返すだけです。

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User Contributed Notes 6 notes

up
88
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
up
11
mattias at kregert dot se
1 year 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
29
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.
up
18
Anonymous
4 years ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
up
4
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 :)
up
-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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