Background / The Problem
I need to make a (large) application with a certain set of core functionality, and certain functionality for multiple clients within it. Each client has different requirements and while they may share some functionality, I need to separate the code for each client, so a change to one won't affect others. Each client section of the application should also inherit some features from the core application. Ideally I would like to separate the databases for each client too. In terms of a user system, core users should be able to have access to each client's functionality, but client users should only be able to interact with their specific client section of the site.
Each client has its own subdomain and the MVC can override the core's MVC files IF they exist. In a nutshell, my custom MVC's autoloader checks the client directory for the matching M,V, or C file when a subdomain is present in the URL and loads that particular client's file INSTEAD OF the core file. It's an override ability in a sense.
Am I solving this problem in a way that would be considered good practice in terms of software design?
If I were to use an existing PHP Framework (I would like to), how might I go about setting it up to solve this problem? Specifically I have been looking at Yii and CodeIgniter.
Thanks in advance!
Update: I have discovered a design pattern called HMVC (Hierarchical MVC) and it seems to be essentially what I have described. I see that Kohana 3 is built using it and that Codeigniter has an extension that allows it.