Youtube pre-Google was still Python, from the way several Youtubers have told it to be. Maybe an initial prototype was PHP, but not what was running when it was purchased.

And trust me, there is no way Google ever would have said "rewrite it in Python." They would have said "rewrite it in Java," like every other frontend they run (Gmail, Calendar, Docs, etc).

my primary reference that it has always been python is this video from 2007 which was only shortly after they were acquired.

http://youtu.be/ZW5_eEKEC28?t=4m8s