Symfony is an MVC framework for PHP 5. It provides an integrated way of creating sophisticated PHP apps.
I originally wrote about Symfony on my use.perl.org blog. Since then, I quit the job where I developed using Symfony, and never tried it again. Symfony had some quirks like the fact that its view component used templates with PHP syntax and privileges, which made it dangerous to let them being customised by the user. Furthermore, its Object-Relational-Mapper (ORM) proved to leak memory, and we had to re-implement some code using it using plain SQL calls. These two issues may have been fixed since then.
Symfony seemed impressive for a PHP-based framework (taking into account the fact that I tend to dislike PHP). We originally decided to convert our web-application to it in order to increase the quality of the code there. However, once I took part in a discussion about a code one of us other co-workers wrote using Symfony. It was MVC-based and all, but still very ugly. This proves that MVC frameworks, while potentially desirable, are not a panacea.