In investigating microframeworks and some of the offerings out there Gonzalo Ayuso has done a little exploring of his own. He's worked up a basic microframework and shared it in a new post as a sort of academic exercise.
Nowadays microframewors are very popular. Since Blake Mizerany created Sinatra (Ruby), we have a lot of Sinatra clones in PHP world. Probably the most famous (and a really good one) is Silex. But we also have several ones, such as Limonade, GluePHP and Slim. Those frameworks are similar.
He looks at how several of these frameworks handle routing and setup, mostly using the closures/anonymous function callbacks available in PHP 5.3. His simple example framework does some basic URI handling to find the requested module, class and function (action) to call. You can even define the output format from options like json, txt, css, js and jsonp. A sample "controller" is included with a "Hello world" and there's a mention of some other options he's exploring including Twig and Assetic integration.