On the Symfony blog they've posted an announcement that they've achieved 100% compatibility with HHVM, the virtual machine/engine created by Facebook, in version 2.3 of the framework.
HHVM is an open-source virtual machine designed for executing programs written in Hack and PHP. HHVM uses a just-in-time (JIT) compilation approach to achieve superior performance for PHP applications. During these last past months, HHVM and the upcoming PHP 7 version have engaged in an epic battle to become the fastest PHP engine. At Symfony we are thrilled because this fierce competition will ultimately benefit all of us.
The post shows some of the commits that were made towards the effort including the first from Joseph Bielawski and the final push from Nicolas Grekas in pull request 15,146 correcting issues in the Debug, DependencyInjection, Filesystem, Form, HttpFoundation, Process and Routing components.