HipHop PHP (was Hyper PHP by Facebook)
The cat is out of the sack now:
http://developers.facebook.com/news.php?blog=1&story=358
HipHop for PHP isn't technically a compiler itself. Rather it is a source code transformer. HipHop programmatically transforms your PHP source code into highly optimized C++ and then uses g++ to compile it. HipHop executes the source code in a semantically equivalent manner and sacrifices some rarely used features — such as eval() — in exchange for improved performance. HipHop includes a code transformer, a reimplementation of PHP's runtime system, and a rewrite of many common PHP Extensions to take advantage of these performance optimizations.
Another article on ReadWrite Web confirms Facebook Gets Faster, Debuts Homegrown PHP Compiler and another one by Marco Tabini describes what you need to know.