They aren't reusing any of the Zend PHP implementation. But that's exactly what phc (http://www.phpcompiler.org) does.
not asking that. is there that much performance diff btw c and c++ or what are the reasons for the choice? first c++ is sexier that's for sure.
From the presentation, it sounded like it generates a C++ object for each user created php object (1 to 1).
We will all know when they release the source.
I suspect they have a lower level c++ framework for speed purposes and they're transition more of the php over. Just a wild guess though.