Zephir is pretty solution than C. Because PHP's internals very terrible and ambiguous with C and macro hell.
There are some questions in my mind about Zephir on long term.
Zephir is a new language and should consume effort to learn and adapt to its behavior. It's not desirable because Zephir looks like domain specific language and it is not universal -for me- currently.
What about portability? There are some processors architectures. What about interoperability? There is a HHVM reality and looks like HHVM going to takeover of PHP's throne in long term.
CPP is universal and too desirable because there are too many developers who known CPP, there are mature toolchains and in active development state by wide society. Also better design to implement structures of PHP's internals. There are overloading and template instead of macro hell. Interoperability with HHVM is too possible because HHVM and HHVM extensions development state by using CPP.
http://www.php-cpp.com/ looks like better implementations of these things.