WordPress accounts for approximately 60% of all websites, PHP accounts for approximately 79% (knows server side sites). So I would say it is particularity popular.
I'd like to know more about that 79% number. I would guess it's actually the case that PHP backends identify themselves as PHP far more than backends in other language x identify themselves.
That doesn't make sense. If you test 100 sites and 79 come back as PHP you can't just claim that the 21 responses that came back blank somehow is a bigger number than 79.
