This is by no means a complete reference to the entire extension and Zend APIs, but it should get you to the point where you can build a simple extension. Through the beauty of open source software, you will never lack example extensions from which to borrow ideas. If you need a feature in your extension that you have seen a standard PHP function do, simply go have a look at how it was implemented. All the built-in features in PHP use the same API.
Once you have gotten to the point where you understand the basic aspects of the extension API and you have questions about more advanced concepts, feel free to post a message to the PHP developers' mailing list. The address is firstname.lastname@example.org. You do not need to be subscribed to send a question to this list. Note that this list is not for questions about developing applications written in user-level PHP. This is a very technical list about the internals of PHP itself. You can search the archives of this list on http://www.php.net by entering a search string in the search field and selecting this list. You can subscribe to this list, and all the other PHP lists, at http://www.php.net/support.php.
Good luck with your PHP extension, and if you write something really cool, please tell us about it on the developers' list!