Best open source project/framework to support a php training course?

I am in charge of outlining a PHP/mySQL course (intro and advanced) for an
education institute. I would like to include in the course program an
in-depth study of an existing open-source project, allowing the students to
be confronted, from an early stage, to a real-world environment. My problem
is simple: which project/framework to choose in order to bring the current
"best practice" to our students, and prepare them for the future. Not being
a professional PHP developer myself, I would appreciate any recommendation.
Which OS project would you include? Which framework? And why?

My first idea was "osCommerce", then somebody recommended "Typo3", followed
by a "go for Joomla". Well, I am hesitating.
Anybody can recommend an article/paper comparing the different PHP
frameworks/application architectures?
Is there a list where such topics are discussed?

Thanks for your help and recommendations.
.