I recently read a new idea of why PHP is useful to people - it's the lowest-overhead server-side Web page language available. If you want a tiny amount of functionality, you write a tiny amount of PHP and don't have to do anything else. So it scales much, much better in the tiny-to-medium-sized project range.

(I can't personally vouch for the claim but it's the best explanation I've heard for how such a reviled language has survived this long.)