Most things you might need to do from PHP are probably already written at extensions to PHP. If you’ve developed a custom API you may need to write your own extension to communicate effectively with your API. There are several excellent books on writing PHP extensions but with the release of PHP5 many of these books are now outdated. A quick search on O’Reilly’s website or on Amazon will yield a variety of reasonable print books and php.net does have some basic documentation on their website. Subscribe to their developer’s mailing list for the most up to date information.