On the Facebook HHVM (HipHop VM) blog there's a recent post sharing some of their progress towards parity with the PHP language inside the tool (and the results of their "three week lockdown").

On November 4th, the HHVM team went on a 3-week performance and parity lockdown. The lockdown officially ended on November 22th. Overall, this lockdown was a qualified success. [...] Going into lockdown, the team knew that awesome performance alone would not suffice in making HHVM a viable PHP runtime to be used out in the wild. It actually had to run real, existing PHP code reliably.

In the post they include some numbers from their testing, the pass/fail status of the unit test suites for several major PHP projects including Composer, Joomla, Laravel, Slim and phpMyAdmin (with an overall parity of 98.58%). They share the raw numbers of the results and describe some of the testing environment, including some "assumptions and caveats" about the process. They also contributed back fixes as a part of the work, putting pull requests out there for several projects. They finish the post with some of the performance numbers, noting that they passed their goal and made it to 16% for an instance of facebook.com.