From the HipHop Homepage:
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
HipHop was developed by Facebook and was released as open source in early 2010.
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.