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Are you looking for a cookie based system or a sessions based system. Sessions require a newer version of PHP (4.1.0 or higher), and cookies don't work well with IE.
Edit: Not to mention, sessions based systems don't last after a person closes the window or leaves the website. I suggest cookies, but you might want a dual system for IE, as IE is a buggered little girl with cookies (Leik OMG, your identity will SOOOOO get stolen from a cookie...)
Session. Its a bit better than the cookie monster technique for me (my version PHP is over 4.1.0) because of the users wanting to disable their yummy cookies
I personally use cookies for my skinning system, but eventually when I get to my new host, I'll put a dual system in for IE, so that IE viewers can visit my website.
Sadly, as I've never used sessions (current host doesn't support it), I'm not quite sure how to use them ^_^.