HipHop Virtual Machine (HHVM) is a process virtual machine based on just-in-time (JIT) compilation that serves as an execution engine for the PHP and Hack programming languages. 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 the x86-64 machine code, optimized, and natively executed. This contrasts to the PHP's usual interpreted execution, in which the Zend Engine transforms the PHP source code into opcodes that serve as a form of intermediate code, and executes the opcodes directly on the Zend Engine's virtual CPU.