There's an interesting blog entry on O'Reilly's web site discussing Digg's PHP Scalability and Performance. As part of his research, the author (Brian Fioca, a Java developer) interviewed Owen Byrne, cofounder and Senior Software Engineer at Digg, and built his own PHP application, Jobby.

Here are some quotes and interesting notes:

• He used the WASP PHP framework.
• Digg gets 200 million page views per month. (Yahoo gets billions per day.)
• Digg uses 3 web servers and 8 database servers.
• "None of the scaling challenges we faced had anything to do with PHP." (Owen Byrne)
• "In fact, we found that the lightweight nature of PHP allowed us to easily move processing tasks from the database to PHP in order to deal with that problem." (Owen Byrne)
• Digg uses APC and MCache.
• "It turns out that it really is fast and cheap to develop applications in PHP." (Brian Fioca)

Note: See PHP Scales for some related content and discussion.