Silex is a PHP microframework for PHP >=5.3
Slim for PHP is a RESTful micro framework for PHP 5 inspired by Sinatra.
I think silex produces a bit uglier code than slim but i m afraid to choose slim because it s changing constantly and i will be stuck with the current version if it breaks backward compatibility
Slim framework vs Silex framework
Silex seems to have a bigger community than slim maybe it s just my point of view
Silex vs SLIM PHP Framework