特殊的 null 值表示一个变量没有值。NULL 类型唯一可能的值就是 null。
在下列情况下一个变量被认为是 null：
被赋值为 null。
尚未被赋值。
被 unset()。
NULL
本特性已自 PHP 7.2.0 起废弃。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。
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 :)
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
Note: Non Strict Comparison '==' returns bool(true) for
null == 0 <-- returns true
Use Strict Comparison Instead
null === 0 <-- returns false
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.
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