And by registration setup i mean.. I've developed a PHP application that i wish to sell. So i need some sort of way to make sure that my system only works if the user is registered / has paid for the app.. This is an area i have no experience in so im looking for advice and best practices. Im thinking an API key that the user only gets when they pay, and the system then validates this key up against a server of mine, when its entered into the app. But should this check happen everytime the app is run or a specific task is run ? Or is there another more appropriate way to do this ?