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.

While not the first attempt to compile PHP code, this comes from one of the largest companies utilizing PHP on the web, so it's one to keep an eye on.

Facebook has rewritten the PHP runtime from scratch. This coming Tuesday, they will make a big announcement around this project, and will make it available as open source software.

Facebook has every reason to have a deep interest in PHP's speed. We'll find out what, if anything, they announce on Tuesday.