Micro-framework (like Flask, or Sinatra) for PHP [closed]

Question

I am working on a single-page Backbone-based app, that uses Facebook's API as a backend mechanism. Therefore, I need to write a very thin glue layer in between, using PHP. I have long-time experiences with CakePHP, CodeIgniter, Kohana, and all the other frameworks from the glorious days of MVC, and I am sure that if I take any of these, I would have to remove a lot of the complexity, rather than benefit from it.

What I am looking for should be a very thin layer on top of vanilla PHP, that gives you all the possibilities to extend, but does not push a bunch of configurations and conventions in your face.

In the Python world, there is a nice micro-framework,called Flask. I believe this is what Ruby's Sinatra also does. So how about a nice microframework in PHP?

Answer 1

Update 2018-05 because this answer still attracts quite some attention. Silex has been discontinued as of January.

There have been talks of getting new maintainers, but as I see it, nothing came of it.

Currently, Slim and Lumen seem like good alternatives, although Symfony v4 can be setup fairly small thanks to the new Flex system.

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Silex is probably the one to go currently. It has the best support of modern PHP programming techniques, is developed in sync with its "big" brother Symfony2, unit-tested, well documented and provides possibility to introduce 3rd party libraries through a very good DI-container mechanism.

Personally, I use this framework for anything that does not need extensive caching, user-managing, heavy database interaction or security. So, basically 95% of the web apps I write.