The special NULL value represents a variable with no value. NULL is the
only possible value of type NULL.
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; ?>
See also the functions is_null() and
Casting a variable to null will remove the variable
and unset its value.