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
