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
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。
语法
null 类型只有一个值，就是不区分大小写的常量 null。
<?php
$var = NULL;
?>转换到 null
警告
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。
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User Contributed Notes 2 notes
quickpick ¶
13 years ago
Hayley Watson ¶
6 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.
