How to limit user from being able to read and copy the content of /opt/lampp/htdocs/* while he can start the PC and launch all the websites from it?

There is a PC which is running an Ubuntu 20.04 LTS OS. It is hosting websites using xampp (Apache + PHP + MySQL). All the setup has been made by a web developer who is a root user. So far, there is a need to distribute this PC to customers who suppose to only be able to benefit from the hosted websites services, but not being able to read, or copy its source code (PHP script). Only a root user should be able to do so. How can this be achieved? Please notice that I'm not a geek in Linux neither in English.