HipHop Virtual Machine (HHVM)
is a virtual machine developed and open sourced by Facebook to process
and execute programs and scripts written in PHP. Facebook developed HHVM
because the regular Zend+Apache combination isn't as efficient to serve
large applications built in PHP.
According to their website, 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 engine + APC (which is the
current way of hosting a large majority of PHP applications).