Well, you can try using cookies. Right before your include do something like:
$user_session_id = session_id();
setcookie("session_id_set","$user_session_id",time()+9999999,"", "mysite.com", "");
(Make sure to substitute mysite.com with your domain, unless it really is mysite.com 🙂
Then in the included file, do:
session_id($HTTP_COOKIE_VARS[session_id_set]);
session_start ();
Worth a try? 🙂
Peace,
Tom