Earlier, nginx was used to handle just static content and passed the php stuff to apache httpd. These days people just use php-fpm instead so nginx can handle everything in a php based stack without needing apache.
Benchmarks on my projects show that the difference is insignificant. Since I currently do not have memory constraints I am still using nginx + apache with mod_php.
But like I said, nginx is great for doing loads of things and I sort of "quit php" recently so I don't really care about php performance anyway.