Facebook has released HipHop Virtual Machine (HHVM) which has 60 percent faster execution engine than its current PHP interpreter and uses 90 percent less memory. I wish official PHP start using this!
Facebook is using this Virtual Machine in its test environment. Facebook says it Just-In-Time complier.
This new HHVM is in addition to HipHop (source code translator) and HPHPi written by Facebook. HPHPi helps developers run their code without compiling it helping developers in debugging. It is slower than HipHop.
More on Facebook's PHP Compiler here.