Awesome topic, superfresh, directly from Scale Conference 2014. The title says everthing. Full quote from video’s description:

As a top 10 website, Wikipedia serves more than 500 million readers ever month. Performance is a serious challenge for the site, particularly when serving users via a legacy PHP application more than a decade old. Any action which bypasses caching, such as editing Wikipedia, takes a serious hit from the limitations of the PHP run time. To solve this problem, we decided to take on the ambitious task of migrating to a new stack centered on HHVM, a virtual machine for PHP with an associated just-in-time compiler. In this talk, Ori Livneh will walk you through the successes and failures we encountered during our migration to HHVM, and share what lessons we’ve derived for anyone seeking to improve performance of PHP applications.