If you're looking for a stripped down, bare-bones look at how to get started with interfacing your PHP script with Facebook, take a look at this quick guide from Emran Hasan.
The new Facebook API has already spread over the application developers and if you’re like me, you’ve already got your hands dirty to see how this new thing works. If you have tried to follow the documentation to authorize/get session in your canvas application, it is likely you have already hit roadblocks. Well, I am no savior but I have glued together a few clues and got it working for myself.
He walks you through the three simple steps: getting the new API, setting ip a correct configuration file and creating a sample "main page" for your application that echoes out a "Hello World" sort of message to each visitor.