Basically what you ask is not possible. The PHP interpreter needs to be able to read the file in order to include it, and if the PHP process can read it then your untrusted user can write some code that would read it in and dump it back out.
A few options you have are:
1) Use an API. Would allow you to keep you code secret as you'd only expose the API. Might take a few days work to implement though (or might not even be possible - impossible to say without knowing what you are doing), so probably not suitable in your situation.
2) Obsfucate your code. There are a number of PHP code obsfucators out there. It wouldn't stop prying eyes completely but it might be enough for your purposes.
3) Create a stub include file. If what your library includes isn't all critical to the running of the code you could create a cut-down stub library for your client to code against, then replace it with the real thing when they've done.