shopping cart rules

Hello,

I am building a shopping cart in PHP and want to know the basic rules
like:

Is the cart associated with a user id? In other words, if a user logs
in from another computer, will his/her cart items still be visible to
him/her?

After the user places an order, do the cart entries get saved in, say,
orders table and get deleted from the cart table? Can we simply just
assign another session id and change the status of the cart table
entries to ordered?

Is there any plac where I can find these and similar rules?

Best Regards,
Asra Baig