NULL

特別な null 値は、ある変数が値を持たないことを表します。null は、null 型の唯一の値です。

変数は、以下の場合に null とみなされます。

  • 定数 null が代入されている場合。

  • まだ値が何も代入されていない場合。

  • unset() されている場合。

構文

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;       
?>

is_null() および unset() も参照ください。

null へのキャスト

警告

この機能は PHP 7.2.0 で 非推奨になります。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 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 :)
up
-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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