One of the key values at Facebook is to move fast. For the past six years, we have been able to accomplish a lot thanks to rapid pace of development that PHP offers. As a programming language, PHP is simple. Simple to learn, simple to write, simple to read, and simple to debug. We are able to get new engineers ramped up at Facebook a lot faster with PHP than with other languages, which allows us to innovate faster. ...HipHop for PHP isn't technically a compiler itself. Rather it is a source code transformer. HipHop programmatically transforms your PHP source code into highly optimized C++ and then uses g++ to compile it. HipHop executes the source code in a semantically equivalent manner and sacrifices some rarely used features — such as eval() — in exchange for improved performance. HipHop includes a code transformer, a reimplementation of PHP's runtime system, and a rewrite of many common PHP Extensions to take advantage of these performance optimizations. Read more: Facebook Developers