PCO is an extension that aims at improve the quickness of PHP by compiling and optimizing the PHP code into machine code. It uses the LLVM librairies, http://llvm.org, which must be installed to your system in order to compile PCO (but is not necessary for working).

For the moment only a very first beta version is available, it uses only a few machine code, but it can already improve the rapidity up to a few percents.
You can find it here, you can also dowload a binary for windows.