I would like to redistribute my app (PHP) in a way that the user gets the front end (presentation) layer which is using the API on my server through a web service. I want the user to be able to alter his part of the app but at the same time exclude such altered app from the normal support and offer support on pay by the hour basis.

Is there a way to check if the source code was altered? Only solution I can think of would be to get check sums of all the files then send it through my API and compare them with the original app. Is there any more secure way to do it so it would be harder for the user to break such protection?