Why does the session id keep changing?

[eluser]ray73864[/eluser]
i am creating a shopping cart system and instead of using the session itself for storing the cart, i store it in the database, after realising that every 5mins the cart was disappearing for some reason, i now create a 'fancy_session_id' which is basically just an md5() of time()+time() 'md5(time()+time())' and use that for the cart, then when the user logs in, i assign the cart to their userid.