The latest interview on the Zend Developer Zone is with Theo Schlossnagle. Theo is probably best known among the PHP community as the founder of OmniTI (where I work), a company that Cal describes as "one of the premier PHP consulting companies in the world." Thanks, Cal. :-)
If you're interested in OmniTI's history or some of Theo's thoughts on scalability, it's worth reading. You might also be interested in Theo's new book, Scalable Internet Architectures.