Resources for plugin based frameworks

Hi, I'm about to start a project and I'm still in the architectural design phase. It's going to be a content management system but for several unrelated things such as website management, employee management or project management. I plan on building a module based system where each part can be developed as an individual plugin and then just added into the portal/main page. Does anyone know of any php-specific resources (either online or paper format, both are fine) that would be helpful? Thank you, Guillaume