The Symfony framework is currently one of the most popular PHP frameworks existing within the PHP developer's environment. Version 2, which was released a few years ago, has been a great improvement, and in my opinion was one of the key elements for making the PHP ecosystem suitable for larger enterprise projects. The framework version 2.0 not only required the modern PHP version (minimal version required for Symfony is PHP 5.3.8), but also uses state-of-the-art technology — namespaces and anonymous functions. Authors also put a lot of efforts to provide long term support and to minimize changes, which break the compatibility between versions. Also, Symfony forced developers to use a few useful design concepts. The key one, introduced in Symfony, was DependencyInjection.
Note
In most cases, the book will refer to the framework as Symfony2. If you want to look over the Internet or Google about this framework, apart from using Symfony keyword you may also try to use the Symfony2 keyword This was the way recommended some time ago by one of the creators to make searching or referencing to the specific framework version easier in future.