HipHop Virtual Machine(HHVM) is an open-source virtual machine based on just-in-time (JIT) compilation that serves as an execution engine for the PHP and Hack programming language. By using the principle of JIT compilation, executed PHP or Hack code is first transformed into intermediate HipHop bytecode(HHBC), which is then dynamically translated into x86-64 machine code, optimized, and natively executed.This contrasts with PHP's usual interpreted execution, in which the Zend Engine transforms PHP source code into opcodes that serve as a form of bytecode, and executes the opcodes directly on the Zend Engine's virtual CPU.
HHVM is developed by Facebook, with the project's source code hosted on GitHub; it is licensed under the terms of the PHP Licenseand Zend License.
What does HipHop do?
HipHop for PHP (HPHPc) is a source to source Compiler. It was created to transfer PHP code into C++, then compile into binary. The open source code is under HP licence, which is in C, C++, and PHP. It was in 2010 when Facebook started working on this project and realised that it could substantially increase the speed of any PHP applications. The figures you can find on the internet claim that the increase in speed is up to six times over the Zend PHP.