Just saying that you can't look at it in isolation.
Low-level php functions are generally wrappers to compiled c+ executables, so yes c++ benchmarked against php 'should' be faster. But factoring in 'how' php is compiled and run, as well as the significantly better i/o and core functionality of linux for multi-threaded environments again 'should' make a difference.
There's a thousand-and-one ways to set things up. The bigger picture is more important.