PHP Modular application

I appreciate you all. the php community is a wonderful community. i`m developing an application that i need to support plugin for additional features, i have used joomla and prestashop. i love the way module interface on prestashop and i have been looking into the classes but i did not understand the theory behind the module positioning. All modules must define at least a position, the position may need to exists inside the template where the module will be displayed. second, assume i have a registration form with 10 Fields matching 10 Fields in a table and later i want to add like 5 more Fields, did i need to alter the table and my scripts again? or a plugin should take care of that, i need some theory to take care of such situation. Thank you in advance.