On the Symfony blog there's a post talking about a tool they've introduced that is helping to "pave the way" for the upcoming version 3 release of the Symfony framework - the Deprecation Detector tool.

Symfony 3 will be released at the end of November 2015. Learning from our own history, the transition from Symfony 2 to 3 will be much more pleasant than the transition from symfony 1 to 2 that happened in July 2011.

Technically speaking, Symfony 3 includes no new features comparing it with Symfony 2.8, which will be released at the same time. [...] This means that your Symfony applications won't work on Symfony 3 unless you remove all their deprecations. In order to simplify the task of finding which deprecations affect your applications, a new tool called Deprecation Detector has just been released.

The tool runs static analysis against your codebase and finds locations where you're using deprecated methods/classes/interfaces/etc and reports them back for fixing. The post includes the commands you'll need to get the tool installed and how to run it against your code. You can find out more about the project and get details on command line options on its GitHub repository.