NULL

NULL 값은 변수에 값이 없음을 표현하는 특별한 값입니다. NULL은 오직 null 타입의 값으로만 가능합니다.

다음과 같다면 변수는 null 로 간주됩니다.:

  • 상수 NULL 이 할당됨.

  • 아직 아무런 값도 지정되지 않은 경우.

  • unset() 된 경우.

구문

null 타입에 대한 값은 오직 하나 대소문자를 구별하지 않는 상수 NULL 밖에 없습니다.

<?php
$var 
NULL;       
?>

다음 함수 is_null()unset() 을 참고 하세요.

NULL 로 변환하기

변수를 null 로 변환하기 위해서는 (unset) $var 와 같이 하면 변수를제거하거나 unset 하지 않고, 오직 NULL 만 리턴할 것입니다.

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User Contributed Notes 6 notes

up
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
6 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
3 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.
up
-8
Mojo
8 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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