With the 400 billion monthly hits Facebook receives, PHP just wasn’t cutting it. Rather than rewrite the entire application into a native programming language, which would alienate their web development staff, Facebook set out to create HipHop, a tool which analyzes PHP code and converts it to C++. For Facebook, this implementation has increased CPU productivity by around 50%, a huge number considering their traffic density and how that will reduce their overall server needs.
HipHop appears to be available as an open source implementation. More information can be found on Facebook’s HipHop announcement page, the HipHop Wiki, or if you’re ready to get started you can join the developers mailing list.