Shopping cart? Ahh!

I have a shopping cart which starts with two categories. The links to each category both have a unique userID so that shopping information can be added to the cart. The problem I have is this: My user can add and remove as many of one category to his cart, but if he links back to the page with both categories listed he loses state (userID) so loses the items in his cart when adding part of the other category. Currently my back button is using this code and I need a quick fix if possible: <? echo "<A href='index_1.php?CA=$CatID&UID=$UID'><img src='../images/nuoli_left.gif' border='0' alt='Back'></a><br><br>";?> Where CatID is the category (currently only one or two) and the UID is the USerID made up of the time date and IP address. You can see what I mean by going here: http://www.violasystems.com/Cart/ and following the links. Click a device server for instance add one of them to the cart and then go back and try adding a starter kit. I want both to appear in the cart if possible. Any ideas? If not I'm off for a cry! Steve Jackson Web Developer Viola Systems Ltd. http://www.violasystems.com stephen.jackson@violasystems.com Mobile +358 50 343 5159