What's so baffling is a behemoth that attracts some of the top engineers would settle for 2 vastly inferior technologies. To be fair, it is a glorified todo / commenting system.
MySQL works pretty well when you treat it as an unreliable network-accessible key/value store; and PHP works pretty well if you treat it as a pure templating language, and also write your own implementation, and also rewrite the language itself to be a different, better language :P