Matthew Weier O'Phinney has been doing some research into PHP microframeworks lately and has shared some of his thoughts in the latest post to his site.

PHP has had microframeworks for quite some time, though I only really first saw the term being used around 3 years ago. The "grand-daddy" of modern-day microframeworks can actually be traced to Ruby, however, and specifically Sinatra. [...] Since around 2009, I've seen an increasing number of new PHP microframeworks4 that follow in the steps of Sinatra and Horde. In the various implementations I've looked at, instead of using a DSL, the authors have all opted for either a procedural or OOP interface.

He gives some code examples showing how the typical routing has been handled in most of the frameworks he's seen and how they handle passing values back to the handler. He sees three use cases for frameworks like this including small, mostly static sites and APIs. He also mentions his efforts to create a microframework using components from the Zend Framework 2 (not released). He finishes up the post with some of his own conclusions about microframeworks and the lessons he learned from his efforts.