For anyone writing a PHP Twitter App to interact with the Twitter API, TwitterOAuth is an incredibly helpful piece of open source code. It handles the secure authentication through OAuth, and then includes function calls for using that authenticated status to carry out any desired GET or POST exchange with the Twitter API.
The project is located at: http://twitter.abrah.am/
The author is pretty careful to remind readers that the library is still in beta, incomplete and likely to change. Nevertheless, his documentation, comments, and live examples make this library a must use.