The framework benchmarks posted repeatedly here have shown PHP can hold its own performance-wise against ruby and python. PHP is not slow. PHP's per request model also makes it easy to build state-free front-end servers which you need in order to scale by throwing more hardware at the problem.

I agree that if you already know another platform there's no reason to switch to PHP, but similarly i've not seen much reason to switch away from PHP, aside from personal preference in language syntax.