HHVM is an open-source virtual machine designed for executing programs written in Hack and PHP. HHVM uses a just-in-time (JIT) compilation approach to achieve superior performance while maintaining the development flexibility that PHP provides.
HHVM runs much of the world’s existing PHP. Developers and hosts are adopting HHVM. We are aware of minor incompatibilities (please open issues when you find them), but we can run the top 20 Github PHP frameworks out of the box. The HHVM team, along with many wonderful community members, has made it a stated, high priority goal to run all existing PHP code existing out in the wild.
The JIT Compiler
Rather than directly interpret or compile PHP code directly to C++, HHVM compiles Hack and PHP into an intermediate bytecode. This bytecode is then translated into x64 machine code dynamically at runtime by a just-in-time (JIT) compiler. This compilation process allows for all sorts of optimizations that cannot be made in a statically compiled binary, thus enabling higher performance of your Hack and PHP programs.