CSS has nothing to do with this sort of thing. Think of CSS like the paint on your walls or the furniture in your house

You are talking about electricity and plumbing here

I would think the only way to achieve this would be by using PHP.

However, I think the cookie system for this is a bit weird!?? I would of thought that something like adding a referrer ID to the url would be much wiser? Maybe one of the PHP guys can enlighten us

I'll move this to the PHP section for you ... you might get more help in there

EDIT! Ah ... Stew beat me to it. I'll leave it here for now then.