Creating extension module for PHP

Hi, I am a newcomer to PHP so I apologise in advance if my question seems trivial or has already been answered on this site before. I have been tasked with creating an HTML based interface for one of our products. We already have a mature API set used to configure the product in the form of a Shared Library on a Mac and a dll on Windows. Is there a means of creating an extension for PHP so that I may use these API calls within a PHP Script? I noticed that there are already a number of extensions available i.e. COM, XML, ODBC etc - can I not create my own extension? Not being a Mac programmer I am not quite sure how these extensions present themselves on the Mac (or even if they exist at all). Any help will be much appreciated. Regards, Simon Fogg.