PHP is used by 76.5% of all the websites whose server-side programming language we know. Alternatives such as ASP.net, Ruby, Java, and JavaScript don’t even come close to this share of the market.

To ease the pain of PHP programmers without making them abandon the language and years of software development, Facebook developed Hack, a new, PHP-derived language that’s largely compatible ...

Basically, VW Phpunit is an extension for PHPUnit, a popular code debugging tool. PHP itself is an immensely popular programming language — most of Facebook's code is written in PHP, for example.