Another very brief article has been posted by Justin Silverton today - this time, it's a "case study" of the hows and whys of Flickr using PHP/MySQL.
Carl Henderson from Flickr.com, a very popular photo blogging service has released a pdf (not sure exactly when this was actually released) detailing the issues they faced with having a high-traffic website.
Based on the original PDF, Justin pulls out some of the key points he noticed as he read. The top level items are "Classes, libraries, and systems used", "unicode support", "Why php was used", and "Mysql usage". He provides more of an outline for each item in the post, touching on the full contents of the formal case study.