Curiously, VKontakte also started from PHP+MySQL but went another way. PHP is compiled ahead of time with something called KittenPHP. It's open-source. For databases they switched some parts to their own bespoke solutions in C that talk with PHP over the memcached protocol. These are called KittenDB (or "engines") for the simple single-purpose ones, and there was also a more generic MySQL replacement in development when I left in 2016, MeowDB.