Read both OAuth 2 (and it's Bearer and MAC specs) and the OAuth 1 RFC.
I would probably avoid reading the PHP code for it. I have a feeling that it's going to do nothing but give you some wrong ideas about how OAuth should be implemented.
I think he meant the OAuth extension for PHP [0] rather than other people's implementations of OAuth in PHP.
Or was that what you meant too? I've not read the OAuth spec yet (though it is on my reading list).
Thank you, Derric Atzrott