HipHop for PHP is a source code transformer for PHPscript code. HipHop programmatically transforms PHP source code into highly optimized C++ and then uses g++ to compile it to machine code.
PHP4Mono is a compiler that enables Mono to run PHP code. It translates PHP scripts to Common Intermediate Language (CIL). Like that PHP scripts can be executed as console applications on Mono. You can even build GUIs directly from within a PHPscript!
PHPCoder is a web based front end to the excellent Turck MMCache and eAccelerator encoding/cache engines. PHPCoder makes it easy to distribute compiled PHP scripts rather than source code and has built in options to allow you to restrict a script.
EncPHP allows you to embed a PHPscript inside of an executable along with everything needed to run the script, including the PHP executable and runtime, and any other DLLs or modules. EncPHP allows you to run PHP apps on computers without PHP installed.