So far as I can find out there are two PHP compilers out there:

Zend Guard, which compiles your code so that it’s optimised, sped up and unreadable. This is run using an Apache / IIS extension called the Zend Optimiser (free).

Phalanger, which is open source and compiles to .NET, allowing the use of .NET classes. It runs on top of the .NET 2.0 framework, so you can run your PHP scripts directly on microsoft windows machines without installing PHP.