The special NULL value represents a variable with 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:
it has been assigned the constant NULL.
it has not been set to any value yet.
it has been unset().
There is only one value of type null , and that is the case-insensitive keyword NULL.
<?php
$var = NULL;
?>
Casting a variable to null will remove the variable and unset its value.