The technology behind Facebook. Among web coders like Zuckerberg, PHP was all the rage in 2003. It gave them a means of building and re-building web software at a particularly fast pace, taking a shortcut around more complicated languages like C++ or Java. But as the months, then the years, passed, PHP’s knack for rapid-fire development would become particularly important to Facebook and The Hacker Way, the philosophy of constant iteration that drives Zuckerberg and his entire company. Facebook engineers like to change things, and change them quickly. PHP lets them do that.

http://www.wired.com/wiredenterprise/2013/06/facebook-hhvm-saga/