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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 null zugewiesen 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;
?>

Siehe auch die Funktionen is_null() und unset().

Umwandlung in null

Warnung

Dieses Feature ist seit PHP 7.2.0 DEPRECATED (veraltet) und seit PHP 8.0.0 ENTFERNT. Von der Verwendung dieses Features wird dringend abgeraten.

Die Umwandlung einer Variable auf den Typ null durch (unset) $var entfernt die Variable nicht oder hebt ihren Wert auf. Es wird lediglich ein null-Wert zurückgegeben.

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User Contributed Notes 6 notes

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95
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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5
hydrogen at live dot in
5 months 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
21
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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17
Anonymous
3 years ago
Note: Non Strict Comparison '==' returns bool(true) for

null == 0 <-- returns true

Use Strict Comparison Instead

null === 0 <-- returns false
up
0
mattias at kregert dot se
2 months 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.
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-8
Mojo
7 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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