Symfony 2 Framework is the best to go. It is idle for medium to big sized projects because it provides clean MVC separation and it is divided into group of packages that are loosely coupled. Also with symfony you can use doctrine which is really good ORM engine (Object Relational Mapping).
I prefer to use Symphony 2 due to its big range of custom's opportunities, you can read more about benefits of this one here - http://webinerds.com/choose-php-application-framework/ If you are an amatour you also can consider Yii Good luck!
So no none here has used, or has any views on Slim as a framework with PHP? I have been told that the new Slim 3 Beta version matches and in many cases exceeds the latest Laravel and Symphony.