I'd agree that there are other languages with equally strong communities and mature libraries. I would say it's easier to hire an OK PHP developer for £40K than for any other language, at least that's my experience hiring in London.

The hosting model - what you said, plus not needing to think about memory leaks. There are so many Java and .NET boxes out there that need to be rebooted every 2 hours because of some insidious bug that nobody on the team can solve. It's also pretty easy to put PHP in a container, and scale horizontally.