I just read about this in Wired -- see link at end of message. Zuckerberg developed Facebook in PHP. As it grew, they needed to do something to improve on PHP, so they invented "Hack". While PHP is dynamically typed, Hack uses "gradual typing": http://ecee.colorado.edu/~siek/gradualtyping.html It runs on Hip-Hop Virtual Machine (HHVM): https://code.facebook.com/posts/264544830379293/hack-a-new-programming-language-for-hhvm/ The open-source version is here: http://hacklang.org/ HHVM is licensed under the PHP and Zend licenses except as otherwise noted. There's already a Wikipedia page for Hack, but there's nothing on it right now: https://en.wikipedia.org/wiki/Hack_(programming_language) According to the article (link below): "Hack will be particularly attractive, Adermann says, to existing PHP shops. “Ironically,” he says, “its chief advantage is how little it differs from PHP.” Like Facebook, these shops can gradually move their operations from one language to the another. But Adermann also believes that some developers will adopt the language even if they’re not already using PHP. “While PHP is the most widely used language on the web, it’s unpopular in many places because of its inconsistencies,” he says. “Hack addresses these … and thereby makes the language more attractive to users of other languages.” "But the biggest endorsement for the new language is that Facebook already uses it to run its own site, the world’s most popular social network. It’s not every day that a new language debuts with such an impressive track record. Some, however, question whether Hack should really be called a new programming language. There’s a fine line here between an update to PHP and a replacement for PHP." http://www.wired.com/wiredenterprise/2014/03/facebook-hack/ Wired 03.20.14 Facebook Introduces ‘Hack,’ the Programming Language of the Future BY CADE METZ