Slack launched in 2014 with a PHP 5 backend, switched to HHVM in 2016 for speed, and later moved to the Hack language with stricter typing. The articles "Taking PHP Seriously", "Hacklang at Slack", and "Hakana" show that a dynamic language from the PHP family can support systems with tens of millions of active users when backed by strong engineering discipline and static analysis. An open example of a mature PHP codebase evolving instead of being thrown away.
Tools
Slack: PHP and Hack at the scale of tens of millions of users
Slack's engineering blog explains why the company built its backend on PHP and later moved to Hack/HHVM for type safety and speed, serving massive concurrency
Our clients
Clients and partners
Which business process it improves
With static analysis and typing, a PHP stack can handle large-scale SaaS load - it is a matter of engineering culture, not a language ceiling.
Open sources