使用PHP进行分层搜索的类结构

I'm just learning object-oriented design and am struggling to see how class inheritance can work in a form I'm doing in PHP. I'm making the search form for an alumni database (for internal use). You choose a graduation period to search a five or ten year interval, then you have radio buttons that allow you to search details about the alumnis' schooling, OR the details of their degree courses, OR what jobs they've had. There are multiple options to refine your search once you've chosen one of the radio buttons.I'm just learning object-oriented design and am