<?php
$a=0;
if (isset($a)) { echo("\$a is set <br />");}
if ($a!==null) { echo ("\$a is not strictly null"); } else { echo ("\$a is strictly null!");}
echo "<br />";
if ($a!=null) { echo ("\$a is not null"); } else { echo ("\$a is null!");}
$a is set
$a is not strictly null
$a is null!