As for the title of your question, sure, PHP/MySQL are stil practical. If the answer to your question were no, then that would mean companies like Facebook and Yahoo aren't practical.
With regard to your future decisions, you might strongly consider another language and framework, not because one is more practical than another, but because the developers behind that framework may have more of a propensity towards the context of your startup. So, for example, Ruby has a large web application community, Python has a large scientific community, and Java enterprise.
The primary contention behind PHP has largely been it's limited OO and speed, as compared to it's brethren interpreted languages. You can, creatively, do advanced things in PHP which are done in other languages, but it's not as straight forward and often clumsy or not supported. That said, PHP is simple enough that large implementations can easily modify PHP for their own use; considering how slow PHP's codebase moves, forward compatibility is far less of an issue.
All together I wouldn't base your decision on speed. Computing power is cheap now and all of them are feasible as far as speed is concerned. If you need to build something NOW, strongly considering going with what you know. If you have more time, consider writing something cliche like a multi-user blog or twitter platform with each framework -- you'll be exposed to patterns you haven't seen before and can also figure out what you like and don't like.