NULL
The special NULL value represents a variable with no value. NULL is the
only possible value of type null.
A variable is considered to be null if:
-
it has been assigned the constant
NULL. -
it has not been set to any value yet.
-
it has been unset().
Syntax
There is only one value of type null, and that is the
case-insensitive constant NULL.
<?php
$var = NULL;
?>
Casting to NULL
Casting a variable to null using (unset) $var
will not remove the variable or unset its value.
It will only return a NULL value.
Коментарии
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 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.