Facebook officially announced the project, titled HipHop, this morning and confirmed that it would be released as open source this evening.

Facebook has been working on a PHP compiler that will increase speed by around 80% and offer a just-in-time (JIT) compilation engine that will offer a number of advantages.

Less CPU means fewer servers, which means less overhead. This project has had a tremendous impact on Facebook

HipHop for PHP isn’t technically a compiler itself, 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.

You can read more about HipHop here