"blackboxing" my global functions

Hello, I act as my own full-service hosting and design company and I use PHP in some of my site designs. I have spent considerable time on some of my functions and such which I generally like to put into an include file which can be accessed by all pages "globally" within the site. What I am wondering is, how can I "black-box" this global function include file such that it is protected from the site "owners" FTP'ing in and downloading/viewing, but is still accessible through the HTTP interactions to pump out HTML to the browser? In a sense I consider the PHP programming I have done my "investment" and "property" and not a part of the design of the site sold to the client, so I seek to protect my investment and keep them from just running off with my code and hosting on a different server. If anyone has any ideas (perhaps I'm missing something obvious as a solution), please advise. Thanks in Advance, ~ Theo