Where can i find docs to create a php5 extensions with OOP as SimpleXML?

Hi, I'm a PHP web programing, but i wanna learn how to develop php5 extensions. I found php5 a good language to apply Objects programing, and it has very usefull examples of OO extensions as SimpleXML, DOM, Sqlite; for this reason i wanna develope my extension following those styles of API's . I've read some docs about creating a php extension that appears as new functions on PHP, but i can't find info to develop extensions that appear as classes on PHP . Thanks for your help. I'm really interested on this topic, especially to collaborate in PHP extension projects as wxPHP.