Well you have not defined what you are trying to do, but if you need to the C++ interface, then look at the ext directory in the source code to understand how to write a PHP extension that you can then load and use from your PHP scripts.
A couple of links that may help:
http://www.devarticles.com/c/a/Cplusplus/Developing-Custom-PHP-Extensions-Part-1/
http://devzone.zend.com/article/1021
