use cookies swift, you dont need php for that, you only need javascript, that is the best way and most easy for you :) no need to register no need to check IP and use php on serverside to track IP downloads etc, just use simple javascript and create cookies with it that you can read, it should take you 15 minutes to understand how to do it with javascript :)
forget IP checking, what happens if there are 10 users at same ip? ;) etc
forget IP checking, what happens if there are 10 users at same ip? ;) etc