The HipHop Virtual Machine (HHVM) is the PHP and Hack execution engine developed at Facebook. HHVM is the fastest existing PHP/Hack engine, and it powers not only Facebook’s servers, but also Wikipedia, Baidu, and many other sites across the web. In this talk, I’ll give an overview of the design of HHVM, with particular focus on its JIT compiler. I’ll share some of our experiences building this high performance system, as well as some of the key challenges that we’ve faced to improve PHP performance.