What are the things that are to be considered to write a framework in php. What should be the structure of the framework. Is there any development guide to write framework for php. How a framework can be written such that it can have api's for the plugins. Like jquery which has a core jquery file(jquery.js) and plugin's that have different jquery file.
NOTE: I know there are some existing frameworks for php. But i want to write it for a specific application.