One component is the fact that it comes out of the box on a great many hosting providers and it's part of the old "standard" LAMP stack. You don't have to set up any reverse proxy crap and the architecture is simple enough - you access a .php file which is really just HTML with some embedded code and it interprets the file. The concept couldn't be any closer to JavaScript which I suspect has attracted so much attention from beginners just because it's everywhere on web clients, just as PHP is everywhere on web servers.