While I don’t think that it’s of vital importance, it could be interesting.
HipHop for PHP: https://github.com/f...ook/hiphop-php/ (I think you’re already aware of its existence)
Unimplemented functions: http://php.webtutor…s-and-features/ (I don’t know if the list is updated)
If full compatibility can’t be achieved, it would be nice to make it as compatible as possibile (i.e. completely drop eval usage etc.) so customization to make it fully compatible would be easier.