According to this new article from the Software Development Times Facebook has rewritten the PHP runtime from scratch and will be releasing it soon as an open source project.

So, why has Facebook rewritten the PHP runtime? Because PHP is obviously too slow for their tastes. A few years ago, I had a coffee meeting with some of the folks from Zend. When they asked what I had been hearing about PHP in the market from my sources, I hemmed and hawed, then told them that I had heard people complaining about how slow PHP was.

Other sources suggest the release won't be a full runtime but instead a sort of compiler for PHP applications including an interview with a Facebook employee who drops a hint about their future plans for the language.