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"
from question "Slim framework vs Silex framework"
"Silex seems to have a bigger community than slim maybe it s just my point of view"
from question "Silex vs SLIM PHP Framework"