Currently works off of the PHP 5.2 language specs
Compiles to a stand-alone libevent-driven web server or CLI executable
Not based on Zend's runtime/code
Currently no windows support
Intended for 'drop in' PHP replacement - if it runs on PHP and doesn't use a few things like eval() it will compile with HipHop
The PHP standard library has a huge number of functions that are mostly useless permutations of one another. As opposed to, say, Erlang's meager but usable lists and string module, PHP has such high-level meta-randomness as stristr() (case-insensitive search), str_word_count(), and str_shuffle(). So if you were going to convert that directly to C++, there wouldn't necessarily be a direct replacement in strings.h. Therefore I'd guess they either have to reimplement all of those in PHP, and compile that to C++, or write them in C++. Furthermore, some of these functions have unusual quirks, so some kind of framework would be required I think.
Still, I imagine they must use a lot of the same code - half the PHP standard library is just a thin wrapper of some C library, and reimplementing all of them sounds stupid.
Then again, they may just not support the majority of such libraries.
This sounds a lot like Python's 2to3. This technique worked so well that the Python 2 series is a distant memory of the past.
(Oh wait... the opposite.)