On the Symfony Finland site they have a post that's celebrating the 5th birthday of Symfony2 and mentions that the latest version, Symfony 3.2, is continuing the legacy and trends the Symfony 2.x releases started.

Late in 2015 the Symfony project as a whole celebrated it's tenth anniversary. Another significant milestone is reached in July 2016 as Symfony2, the second major iteration celebrates it's fifth anniversary. Five years is a long time in IT, but even more so in web development.

[...] Symfony2 was a major rewrite that turned the previous monolithic framework into PHP components as well as a framework to tie these into a coherent framework for developers to build their applications on. Since that Symfony3 was released and was more of an evolutionary release.

He looks back at Symfony2 in hindsight and features/functionality it brought to the table that made major impacts in the PHP ecosystem. This includes things like heavy use of dependency injection, bundles and a standardized interface on HTTP requests and responses. He ends the post looking at some of the new features that are coming with Symfony 3.2, listing out several links back to Symfony's own blog posts with more details on each.