Every application needs to tackle scalability in its own way. Yes- you can use PHP to run a very busy web site, but you may need to take steps to optimise things which you wouldn't normally do.
Build the application first in the most logical well designed way, and worry about optimising it later.
90% of clients who claim that their site is going to be incredibly busy never actually market it correctly etc anyway, so don't get their massive levels of predicted traffic anyway.
Don't optimise prematurely.
Mark