"The short development cycles needed to stay ahead of the competition demand a web-centric scripting language that is easy to maintain and update."
http://public.yahoo.com/bfrance/radwin/talks/yahoo-phpcon200...
They didn't choose Python, or Ruby, or Java, because none of those languages are as expressive or shut-up-and-get-out-of-my-way as PHP. Just because a high schooler can write PHP doesn't mean it's a toy not also meant for world class engineers.
