How to protect my php code running on a client server?

Question

I'd like to ask how to protect my code that is running on a client server from being altered and/or copied.
It's running on Ubuntu Server with the standard Apache Php Mysql stack.
Any software needed would preferably be open-source.

I'd also like to terminate the application when there is a license problem.
Doing this using my code is possible but since it's php the client could alter the code to prevent this from happening.