PHP engineer of 10 years here - all things being equal, I cannot, in good faith, recommend a person use PHP for anything. General rule of thumb is, if you can, use Python. We still see folks actively choosing it though.. mostly because it's a language we already know and better to create an MVP in something folks know rather than struggle with something "better" that will take a person 5x longer to write while they learn it.

I would bet a donut that Slack is off PHP within 2 years.