La valeur spéciale NULL représente une variable sans valeur. NULL est la seule valeur possible du type null.

Une variable est considérée comme null si :

  • elle s'est vue assigner la constante NULL.

  • elle n'a pas encore reçu de valeur.

  • elle a été effacée avec la fonction unset().


Il y a une seule valeur de type null, et c'est la constante insensible à la casse NULL.


Voir aussi les fonctions is_null() et unset().

Transtyper vers NULL


Cette fonctionnalité est OBSOLÈTE à partir de PHP 7.2.0. Dépendre de cette fonctionnalité est fortement déconseillé.

Transtyper une variable vers null en utilisant la syntaxe (unset) $var n'effacera pas la variable, ni écrasera sa valeur. Ca ne fera que retourner la valeur NULL.

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User Contributed Notes 5 notes

8 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
1 year ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
Hayley Watson
1 year 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.
kuzawinski dot marcin at NOSPAM dot gmail dot com
5 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
11 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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