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。


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。


is_null() および unset() も参照ください。

NULL へのキャスト


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

9 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
Hayley Watson
2 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.
2 years ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
kuzawinski dot marcin at NOSPAM dot gmail dot com
6 years ago
Funny. It looks like, that there is one, and only one possible value for variable $a that will pass this test:

($a != NULL) && ((bool)$a == NULL)

It's "0" and it works because casting string "0" to boolean gives FALSE (and it's the only non empty string, that works this way). So remember that casting is not "transitive".
nl-x at bita dot nl
12 years ago
Watch out. You can define a new constant with the name NULL with define("NULL","FOO");. But you must use the function constant("NULL"); to get it's value. NULL without the function call to the constant() function will still retrieve the special type NULL value.
Within a class there is no problem, as const NULL="Foo"; will be accessible as myClass::NULL.
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