Version 2 of the Silex PHP Microframework has been out for a while, but many sites are still using Silex version 1. Perhaps this is because of Silex 2's backwards-compatibility breaks.
The Changelog file on the Silex website helpfully touches on these changes, but it can be tough to understand how to translate the entries into meaningful code. I've found the documentation on the Silex 2 website to be spot-on, but it's not always clear when the examples are different from Silex 1 code. To that end, I've started writing a wiki page on the Silex project that should help clarify things. A portion of it is presented here (based on a blog post I started writing over a year ago and forgot to finish. Oops!).