John Hawkins, owner of Vegas-based 9seeds (http://9seeds.com/) and the man behind vegasgeek.com (http://vegasgeek.com/) will describe the architecture of WordPress and will show how plugins and themes are developed in PHP.
Per Wikipedia (http://en.wikipedia.org/wiki/Wordpress):
WordPress is a free and open source (http://en.wikipedia.org/wiki/Free_and_open_source) blogging (http://en.wikipedia.org/wiki/Blog) tool and a dynamic content management system (http://en.wikipedia.org/wiki/Content_management_system) (CMS) based on PHP (http://en.wikipedia.org/wiki/PHP) and MySQL (http://en.wikipedia.org/wiki/MySQL). It has many features including a plug-in architecture and a template system. WordPress is used by over 14.7% of Alexa Internet (http://en.wikipedia.org/wiki/Alexa_Internet)'s "top 1 million" websites and as of August 2011 manages 22% of all new websites. WordPress is currently the most popular CMS in use on the Internet.
John's company 9seeds is a custom WordPress development shop. John has done many WordPress presentations at many conferences over the years (http://vegasgeek.com/speaking/), the most recent being on the business aspects of WordPress plugin development (http://vegasgeek.com/business-aspects-of-wordpress-plugin-development/).