ZEND modular application
It may be a little large, but Magento is based on Zend Framework and is very modular. The Zend classes used in it are slowly being replaced by Magento-specific ones, but they still take up a lot of Zend Framework elements such as controllers, helpers, models.
They offer configuration for each module to connect to the system router, administrator, templates (you may not even have to edit the template file). You can also override models, controllers, and other modules using module configuration.
To be honest, I haven't looked that much, but when it comes to extensibility: Magento is hard to beat. To use it as a base for any other system, there must be improvements in other areas such as performance, scalability, bugs, complexity, and compatibility.
a source to share
One idea is to go to the website for the excellent book " Zend Framework 1.8 Application Development " and download the source code for the book (which is freely available). The application completed in Chapter 12 is modular and has many infrastructure features as well as many great development techniques, so I can only assume that familiarity with such an application will be beneficial to you.
a source to share
How about Zend Framework in action by Rob Allen? This is a great book, and Rob has a very simple style and explains a lot of topics in a very straightforward manner.
Alternatively, clone a copy of my Zend Framework development project from github. I am writing this specifically to cover all the different concepts available in the Zend Framework; it uses modules.
All the best
Matt
a source to share