Roadsend PHP compiler: "an entirely new implementation of PHP that is syntactically and symantically compatible with PHP Group's PHP 4.3.x and the Zend Engine 1"

Quercus: "Caucho Technology's fast, open-source, 100% Java implementation of the PHP language."

php4mono: doesn't support "include". On the roadmap is "will implement a basic library containing standard functions available in PHP"