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 类型是 PHP 的原子类型（unit type），也就是说，它仅有一个值 null。
未定义和 unset() 的变量都将解析为值 null。
null 类型只有一个值，就是不区分大小写的常量 null。
<?php
$var = NULL;
?>null
本特性自 PHP 7.2.0 起废弃，并且自 PHP 8.0.0 起被移除。 强烈建议不要使用本特性。
使用 (unset) $var 将一个变量转换为 null
将不会删除该变量或 unset 其值。仅是返回 null 值而已。
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 falseNULL 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.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 :)Note: Non Strict Comparison '==' returns bool(true) for
null == 0 <-- returns true
Use Strict Comparison Instead
null === 0 <-- returns falseNote 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.