入口文件 index.php

一、 运行流程

1. The index.php serves as the front controller, initializing the base resources needed to run CodeIgniter.
2. The Router examines the HTTP request to determine what should be done with it.
3. If a cache file exists, it is sent directly to the browser, bypassing the normal system execution.
4. Security. Before the application controller is loaded, the HTTP request and any user submitted data is filtered for security.
5. The Controller loads the model, core libraries, helpers, and any other resources needed to process the specific request.
6. The finalized View is rendered then sent to the web browser to be seen. If caching is enabled, the view is cached first so that on subsequent requests it can be served.

二、入口文件

　　1、定义当前运行环境，并设置错误级别

      2、定义系统文件路径 $system_path、应用目录 $application_folder、视图目录 $view_folder ， 并格式化 realpath

      3、 定义常量：

• •  SELF 入口文件名 
  •  BASEPATH 系统目录
  • FCPATH     入口文件所在目录
  • SYSDIR    系统文件名 system
  • APPPATH
  • VIEWPATH

三、引用核心文件  core/CodeIgniter.php