On PHPBuilder.com there's a recent post showing you how to connect your application with Facebook's graph API and grabbing the current user's public profile information.

In recent years, [Facebook's] influence has dramatically grown thanks to the Facebook Platform, a set of APIs which third-parties can use to create or extend applications which tightly integrate with Facebook.com's features and users. [...] PHP-minded developers are particularly fortunate, as the Facebook PHP SDK doesn't only provide users a powerful solution for interacting with the social graph, but because it's actively maintained by the Facebook development team is often the first of several available APIs to offer the latest features and bug fixes.

He points out the github repostory for grabbing the Facebook SDK, the information you'd get (at a minimum) from the API and the sort of detail you can expect from a user logged into your application. Sample code is included for this last example.