I originally found this link in a news post on the php|architect site. (You can find the news post a few pages back in their archive).

It is a link to speaking notes of Michael J. Radwin from Yahoo talking about one year of PHP use at Yahoo.
http://public.yahoo.com/~radwin/talks/

There are occassional threads about "is PHP Enterprise ready?" - Well, given the size of Yahoo, I think they make an interesting point of reference for any debate.

Cheers,
Keith.