Making Php Libraries

I am new to this list so forgive me if this has already been addressed. What I would like to do is keep a library of php scripts on my site. I would like customer sites to call the functions in my library, but have their server do the work. I would like to keep the proprietary code off of their site. I have been able to do this by changing the extension of the library file from php to inc. Unfortunately, this also makes the library file readable if the user puts the url for the file into the browser. Thanks. Chris Taylor