HHVM is a new open-source virtual machine designed for executing programs written in PHP. HHVM uses a just-in-time compilation approach to achieve superior performance while maintaining the flexibility and ease of use that PHP developers are accustomed to (dynamic features like eval(), rapid run-edit-debug cycle, etc).
HHVM is used by Facebook to serve billions of web requests per day. To date, HHVM has realized over a 9x increase in web request throughput and over a 5x reduction in memory consumption for Facebook compared with the Zend PHP 5.2 engine + APC.