Allan MacGregor has posted a (high level) introduction to the HipHop Virtual Machine (HHVM) that was created by Facebook to address some of their PHP-related needs inside their platform. It sacrifices some things in the name of speed and less resource consumption.
In early 2008 Facebook began working on HipHop(now HPHP), a PHP execution engine; its original motivation was to convert Facebook massive PHP code base into C++ in order to save resources and increase the application performance. [...] At it’s peak, HipHop PHP code showed up to 6x times better performance than its ZEND counterpart. However, there where several drawbacks to this first iteration of HipHop. [...] At the same time Facebook started the development of the modern version of HipHop, known as HHVM (HipHop Virtual Machine), HHVM improves the strengths of HPHPc and corrects many of the critical problems.
He talks some about Facebook's efforts to open source the platform and what the HipHop VM does to make your PHP code execute that much faster. It's all about bytecode, machine code and a JIT (just in time) compiler where the bytecode is parsed during execution instead of before.
Currently HHVM supports PHP 5.4 almost on its entirety, however there are still numerous bugs that prevent some applications from running, for that reason Facebook has set as goal to have the top 20 open source PHP applications running on HHVM. The first popular application to achieve this was Wordpress.