Session ID changes

Hello all, I need some clarification. To my understanding, each visit to php site creates a UNIQUE Session ID (SID). And, that ID stays constants until the browser is shutdown, or the session is specifically destroyed. Is this correct? If not, then please advise. I'm trying to develop a cookie-less, persistent, shopping cart, which saves the cart details, in a database. I need to associate each order (cart item) to the current session id. However, the session ID number, seems to change, everytime the page is reloaded. Is this how it works? Am i missing something? Please advise. Thanks. -john