Session for creating a unique shopping cart for each user

Hi, I am creating a shopping cart but experiensing some difficulty in creating unique shopping carts for each user. When I try to create a session I get this error message: Warning: session_start(): Cannot send session cookie - headers already sent This is the code: if(isset($_COOKIE["cartId"])) { return $_COOKIE["cartId"]; } else { session_start(); session_register("cartId", session_id(), time() + ((3600 * 24) * 30)); return session_id(); } If I remove the session_start() and session_register it works fine, but then everyone uses the same cart and that can't be good. Thanx!!! Regards Tore