Domain validation

This is for an existing flash chat room - I would like to limit it per domain when I sell it. On chat load, it should communicate with a PHP (creating XML or whatever) file to check weather the current domain is licensed. for example: the chat is located in [login to view URL] when a visitor goes there, it will check against the main server with something like that: [login to view URL]?url=[login to view URL] If it's valid, allow it to work and place a cookie for 1 hour. If not valid, display a popup. If no communication (main server is down) - allow it to work.