Roadsend's PHP Compiler is now open source!
It moves PHP into directions never possible before, by compiling PHP into native executables on Linux, FreeBSD and Windows. It does not depend on Zend's PHP engine at all.
It has some drawbacks though: it requires Scheme to compile it (more work), does not support PHP 5 (stuck with PHP4), and has no 64-bit version yet (my test server is AMD64 native 64bit).
After compiling your PHP application, you can run it using the built in microserver, or as a FastCGI process.
Read the FAQ on Roadsend's web site.
Thanks to Arto Bendiken's pointer in another thread.