This has long been a brief excursion into the world of writing PHP extensions, but I hope I've shown you that really it's not such a hard thing to do. Sure, the macros take a bit of learning, but helper functions like zend_parse_parameters() can really lower the learning curve.
The best way to learn how to use the PHP extensions system is just to pick a problem you want to solve and dive right in. So, what are the bottlenecks in your code? Would it work if it were moved to C? Have fun!