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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).
wget -O - http://dl.hhvm.com/conf/hhvm.gpg.key | sudo apt-key add -
echo deb http://dl.hhvm.com/ubuntu trusty main | sudo tee /etc/apt/sources.list.d/hhvm.list
sudo apt-get update
sudo apt-get install hhvm