FaceBook has put its efforts into building quicker PHP execution engines with the HipHop for PHP in 2010 for converting PHP code into native code to the new HipHop virtual machine that compiles to HipHop byte code. The source code is being shared by FaceBook on GitHub and has a mailing list and a bug tracker for the project as well.

Look up the FaceBook note on HipHop for more information.