ilia.ws
My Thoughts on HipHop
In the tersest of terms HipHop is a tool that converts PHP code into C++ code that when combined with a PHP compatible engine and extensions (ports of some native PHP extensions Facebook uses) library also written in C++ can be compiled using GCC into a binary. This binary can then be ran on a command line or as a web server daemon that utilizes libevent. According to Facebook this can speed up applications by up to 50%, which is a pretty impressive improvement.