Types
PHP Manual

NULL

The special NULL value represents that a variable has no value. NULL is the only possible value of type NULL.

Note: The null type was introduced in PHP 4.

A variable is considered to be NULL if

Syntax

There is only one value of type NULL, and that is the case-insensitive keyword NULL.

<?php
$var 
NULL;       
?>

See also is_null() and unset().


Types
PHP Manual