Use this code at the top of the php page, should fix the cache problem (I use it in my dynamic signature)
Code:
header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
header('Cache-Control: no-store, no-cache, must-revalidate');
header('Cache-Control: post-check=0, pre-check=0');
header('Pragma: no-cache');