Shopping Cart Schema - Sessions

Hey, I'm looking for little information from those who have coded their own shopping cart apps. what would you say the best way to setup a cart would be? more specificly, the method for adding items to the cart. say the user is broswing around. when they entered the website a new session was created. when they go to add an item, what is the best way store variables (such as what they bought and quantity) in the session. i have read that it is possible to put all of your session variables in a single associative array. however, i have not had any luck doing this myself. Under such a method of an array, how would i store each item (product id and quantity, possibly price as well) in the cart into the session. obviously i don't expect anyone to write an entire app. i am comfortable starting the session, and doing th db extraction and presentation. i just need to know how to store a variable in a session (passed through post) and how to extract it when need be (considering it is an array). Thanks a lot!