Although we have experience developing PHP/MySQL websites from scratch, some of which are still in production, all our new development is Drupal-based. Drupal allows very rapid, low-cost development and there is a very large, worldwide, self-supporting user-base. The Drupal.org server is hosted by Oregon State University in Corvallis.
Drupal is an open-source (free) platform and content management system for building dynamic web sites offering a broad range of features and services including user administration, publishing workflow, discussion capabilities, news aggregation, metadata functionalities using controlled vocabularies and XML publishing for content sharing purposes. Equipped with a powerful blend of features and configurability, Drupal can support a diverse range of web projects ranging from personal weblogs to large community-driven sites. You can read more on the About Drupal page or better yet, check out this 5-minute video overview: