入口：index.php

处理各种环境变量

APPLICATION ENVIRONMENT

CUSTOM CONFIG VALUES

main path constants

定义完成后，启动bootstrap file

system/core/CodeIgniter.php

defined('BASEPATH') OR exit('No direct script access allowed');  非框架访问则拒绝

framework constants

global functions     目前比较重要 load_class   is_loaded  get_config

custom error handler

Start the timer... tick tock tick tock...

Instantiate the hooks class

Instantiate the config class

Instantiate the URI class

Instantiate the routing class and set the routing

Instantiate the output class

Load the base controller class

Instantiate the requested controller

Call the requested method

Send the final rendered output to the browser