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().
There is only one value of type null, and that is the
case-insensitive constant NULL.
<?php
$var = NULL;
?>
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.