1. index.php 文件作为前端控制器，初始化运行 CodeIgniter 所需的基本资源；
2. Router 检查 HTTP 请求，以确定如何处理该请求；
3. 如果存在缓存文件，将直接输出到浏览器，不用走下面正常的系统流程；
4. 在加载应用程序控制器之前，对 HTTP 请求以及任何用户提交的数据进行安全检查；
5. 控制器加载模型、核心类库、辅助函数以及其他所有处理请求所需的资源；
6. 最后一步，渲染视图并发送至浏览器，如果开启了缓存，视图被会先缓存起来用于 后续的请求。

1.设置应用运行环境常量

    define('ENVIRONMENT', isset($_SERVER['CI_ENV']) ? $_SERVER['CI_ENV'] : 'development');

2.设置报错级别

switch (ENVIRONMENT)
{
    case 'development':
        error_reporting(-1);
        ini_set('display_errors', 1);
    break;

    case 'testing':
    case 'production':
        ini_set('display_errors', 0);
        if (version_compare(PHP_VERSION, '5.3', '>='))
        {
            error_reporting(E_ALL & ~E_NOTICE & ~E_DEPRECATED & ~E_STRICT & ~E_USER_NOTICE & ~E_USER_DEPRECATED);
        }
        else
        {
            error_reporting(E_ALL & ~E_NOTICE & ~E_STRICT & ~E_USER_NOTICE);
        }
    break;

    default:
        header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
        echo 'The application environment is not set correctly.';
        exit(1); // EXIT_ERROR
}

3.设置系统目录常量

    $system_path = 'system';

    if (defined('STDIN'))
    {
        chdir(dirname(__FILE__));
    }

    if (($_temp = realpath($system_path)) !== FALSE)
    {
        $system_path = $_temp.DIRECTORY_SEPARATOR;
    }
    else
    {

        // Ensure there's a trailing slash
        $system_path = strtr(
            rtrim($system_path, '/\\'),
            '/\\',
            DIRECTORY_SEPARATOR.DIRECTORY_SEPARATOR
        ).DIRECTORY_SEPARATOR;
    }

    // Is the system path correct?
    if ( ! is_dir($system_path))
    {
        header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
        echo 'Your system folder path does not appear to be set correctly. Please open the following file and correct this: '.pathinfo(__FILE__, PATHINFO_BASENAME);
        exit(3); // EXIT_CONFIG
    }

    // The name of THIS file
    define('SELF', pathinfo(__FILE__, PATHINFO_BASENAME));

    // Path to the system directory
    define('BASEPATH', $system_path);

    // Path to the front controller (this file) directory
    define('FCPATH', dirname(__FILE__).DIRECTORY_SEPARATOR);

    // Name of the "system" directory
    define('SYSDIR', basename(BASEPATH));

4.设置应用常量目录

    $application_folder = 'application';


    // The path to the "application" directory
    if (is_dir($application_folder))
    {
        if (($_temp = realpath($application_folder)) !== FALSE)
        {
            $application_folder = $_temp;
        }
        else
        {
            $application_folder = strtr(
                rtrim($application_folder, '/\\'),
                '/\\',
                DIRECTORY_SEPARATOR.DIRECTORY_SEPARATOR
            );
        }
    }
    elseif (is_dir(BASEPATH.$application_folder.DIRECTORY_SEPARATOR))
    {
        $application_folder = BASEPATH.strtr(
            trim($application_folder, '/\\'),
            '/\\',
            DIRECTORY_SEPARATOR.DIRECTORY_SEPARATOR
        );
    }
    else
    {
        header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
        echo 'Your application folder path does not appear to be set correctly. Please open the following file and correct this: '.SELF;
        exit(3); // EXIT_CONFIG
    }

    define('APPPATH', $application_folder.DIRECTORY_SEPARATOR);

5.设置视图路径常量

    $view_folder = '';

    // The path to the "views" directory
    if ( ! isset($view_folder[0]) && is_dir(APPPATH.'views'.DIRECTORY_SEPARATOR))
    {
        $view_folder = APPPATH.'views';
    }
    elseif (is_dir($view_folder))
    {
        if (($_temp = realpath($view_folder)) !== FALSE)
        {
            $view_folder = $_temp;
        }
        else
        {
            $view_folder = strtr(
                rtrim($view_folder, '/\\'),
                '/\\',
                DIRECTORY_SEPARATOR.DIRECTORY_SEPARATOR
            );
        }
    }
    elseif (is_dir(APPPATH.$view_folder.DIRECTORY_SEPARATOR))
    {
        $view_folder = APPPATH.strtr(
            trim($view_folder, '/\\'),
            '/\\',
            DIRECTORY_SEPARATOR.DIRECTORY_SEPARATOR
        );
    }
    else
    {
        header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
        echo 'Your view folder path does not appear to be set correctly. Please open the following file and correct this: '.SELF;
        exit(3); // EXIT_CONFIG
    }

    define('VIEWPATH', $view_folder.DIRECTORY_SEPARATOR);

5.加载核心控制文件

/*
 * --------------------------------------------------------------------
 * LOAD THE BOOTSTRAP FILE
 * --------------------------------------------------------------------
 *
 * And away we go...
 */
require_once BASEPATH.'core/CodeIgniter.php';