Web server files. How to secure them?

In my company, we recently developed a web application using WAMP. I know it doesn't seem very secure and it is not, but it worked for us since we only use it on our private network.

Now some company want to buy it from us and I could do the same, set it up using WAMP on one of their computer (server role) but the thing is I don't want them to be able to see/change/copy our .php files.

Is there any way I could accomplish this? I prefer a solution using WAMP but it is not necessary. Thanks.

Edit: I have to mention that they want to have their own server there, locally.