Keith Adams, Chief Architect at Slack, gives some background on how PHP is used at his company.

Slack uses PHP for most of its server-side application logic, which is an unusual choice these days. Why did we choose to build a new project in this language? Should you? Most programmers who have only casually used PHP know two things about it: that it is a bad language, which they would never use if given the choice; and that some of the most extraordinarily successful projects in history use it. This is not quite a contradiction, but it should make us curious. Did Facebook, Wikipedia, Wordpress, Etsy, Baidu, Box, and more recently Slack all succeed in spite of using PHP? Would they all have been better off expressing their application in Ruby? Erlang? Haskell?

https://slack.engineering/taking-php-seriously-cf7a60065329