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Pro Built on top of Symfony components
The thing that makes Silex stand out from other PHP microframeworks is the fact that it's built using some of Symfony2's components. Making it quite powerful but still lightweight enough to be considered a true microframework.
Pro Open source
Silex is open source and is licensed under the MIT license.
Pro Simple and elegant DI container
Based on Pimple, Silex has a simple Dependency Injection container that consists of just one file and one class.
Silex makes use of Symfony2's HttpKernel which is used to abstract requests and responses. This in turn, makes it very easy to test apps created with the framework.
By using Pimple, the Silex application extends the Pimple class, which in turn is nothing more than an implementation of the ArrayAccess interface that has been a part of PHP since version 5.0.
This makes it possible to use an instance of the Application class as if it were an array. Like so:
$app = new Silex\Application();
$app['config'] = new Config($config_path);
This gives developers a great deal of flexibility when injecting dependencies and when testing.
Con Documentation is not very extensive
The documentation for this framework is average, it's helpful mostly. But it is not extensive enough to cover everything in detail unfortunately.