PHP Manual: NULL

NULL

The null type is PHP's unit type, i.e. it has only one value: null.

Undefined, and unset variables will resolve to the value null.

There is only one value of type null, and that is the case-insensitive constant null.

<?php
$var = NULL;       
?>

Warning

This feature has been DEPRECATED as of PHP 7.2.0, and REMOVED as of PHP 8.0.0. Relying on this feature is highly discouraged.

Casting a variable to null using (unset) $var will not remove the variable or unset its value. It will only return a null value.