Extending PHP with custom modules?

Hi, I've been researching PHP for a while now, and it seems to be a great language. However, there seems to be no well-documented way to add native functionality, i.e. custom modules, to PHP. I saw the "PHP Development" section in the docs, but that didn't seem to explain how to link the new modules into the library, or how to make it visible to the PHP engine. Glancing through the code, I noticed that you are supposed to create a function table and a module structure which describe the functions available, but I could not find out how to link that to the engine. Also, how does the PHP threading structure work? Any explanations or pointers to resources would be greatly appreciated! Thanks, David Xiao