PHP continues to thrive because of WordPress and PHPBB. There are so many sites running these two pieces of software that PHP will be a top language for years. PHP is also one of the easiest languages for Designers to learn. If you know HTML, adding a few bits of PHP is easy. Even if you dislike PHP, (I certainly do) you end up getting good at it because enough projects touch it that you kind of have to be good at it…
In 10+ years of professional web development, I've had, I think, 1 client ever who had PHPBB on their site, but I work on at least 1-5 sites a month that are running a variation of WordPress (or just PHP in general). And every new site we setup is running WordPress on the back-end now.
I've also never had a client come to me who was running a Ruby, Python, etc, system. The strangest it has gotten was a Cold Fusion site here and there, and maybe once a year a .Net / ASP site crawls through the door..... and we have pretty religiously completely converted the sites to PHP in these cases before we start to manage them.