I've got a similar situation where I want others to call my php.
I'm going to give them a url and variables to call and return a blank page with a form and two hidden fields (status and a validation code).
The values in the hidden fields is their output. If they needed more I'd just add extra hidden fields.
The beauty of this is I don't have to install anything extra, I don't have to worry about what cgi they use etc. Plus the url they call will call other functions so they don't get visibility of my important code.
HTH
Sarah