Forms submitting to a session array

I'm trying to create a shopping cart where users can visit a product page (called from a MySQL database depending on the url ending in ?product_id=#). On the product page users can select options from dropdown lists (such as color, etc.) These dropdown lists are also dynamically generated from the database. I need to store what the user has selected while the user browses the site. I want to also display what they've selected on a "cart" page. I believe my best option is to store what they've selected in a session array. I'm not sure how to do this with the dropdown lists, etc. Any help would be greatly appreciated! -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/