HipHop for PHP
HipHop for PHP transforms PHP source code into highly optimized C++. It was developed by Facebook and released as open source in early 2010.
HipHop transforms your PHP source code into highly optimized C++ and then compiles it with g++ to build binary files. You keep coding in simpler PHP, then HipHop executes your source code in a semantically equivalent manner and sacrifices some rarely used features - such as eval() - in exchange for improved performance.
Facebook sees about a 50% reduction in CPU usage when serving equal amounts of Web traffic when compared to Apache and PHP. Facebook's API tier can serve twice the traffic using 30% less CPU.