Thanks for the tip. I'll look into Kohana. I've been considering learning Zend Framework in part because there seems to be a lot of documentation and books about it. I'm big on books.
But really I'm looking for a complete working project. The areas I'm particularly interested in studying are a full-featured user management system, security, & code organization. The other downside of the PHP projects I studied in books is that they were basically just PHP, HTML, & CSS. They didn't contain any Javascript, AJAX, XML, etc. I would like to find a project that contains all of these technologies. I'm familiar with these technologies already, but I just feel like I could use some help with organization & design.
I was considering Zen Cart but now I'm thinking it may be a little too complex for me. Thoughts?