Calling c function from php

hello, I thought I posted this yesterday but accidentally posted to php-db. Anyway, can any make a suggestion on how to connect to a c function from a php app? I have to connect to a sales tax calculation software. The function is being written for me but I have to figure how to call it - get the variable - and use it again in my php app. I've been told there's an api or module on the php site with documentation that I am trying to research. Has anybody ever done this ? Thx -- M Hunter