Looping through a list - Newbie question

Hi, I have a list contained in a session var. I want to loop through the list, getting the value of the current item, then do a query based on that value. Is there an easy way to do this, or do I need to convert the list to an array first? In the code below, $id isn't being set. Here's my code (this is just testing stuff, sv_CampusList is set somewhere else): <?php session_start(); $_SESSION['sv_CampusList'] = "1,2,4,5"; for($i=1; $i<=strlen($_SESSION['sv_CampusList']); $i++){ $id = strpos($_SESSION['sv_CampusList'],$i); echo $id; } ?> Thanks, James