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
NULL
Der spezielle Wert NULL repräsentiert eine Variable ohne Wert.
NULL ist der einzig mögliche Wert des Typs null.
Eine Variable gilt als vom Typ null wenn:
-
ihr die Konstante
NULLzugewiesen wurde. -
ihr noch kein Wert zugewiesen wurde.
-
sie mit unset() gelöscht wurde.
Syntax
Es gibt nur einen Wert vom Typ null:
die Konstante NULL (Groß- und Kleinschreibung ist
dabei nicht wichtig).
<?php
$var = NULL;
?>
Umwandlung in NULL
Warnung
Dieses Feature ist seit PHP 7.2.0 als DEPRECATED (veraltet) markiert. Sich auf dieses Feature zu verlassen ist in keiner Weise empfehlenswert.
Die Umwandlung einer Variable auf den Typ null durch
(unset) $var
entfernt die Variable nicht und löscht nicht ihren Inhalt.
Es wird lediglich ein NULL Wert zurück gegeben.
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User Contributed Notes 5 notes
quickpick ¶
9 years ago
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.
Anonymous ¶
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 ¶
13 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.
