CI框架源码解读(1)-入口文件index.php

参考：http://www.cnblogs.com/ohmygirl/p/CIRead-2.html

入口文件是进入一个框架的开始，下面分析下CI入口文件。

CI将入口文件放在根目录，而不是像yii或laravel或新版本TP将入口文件放到根目录下一个文件中，原因和禁止直接访问文件的处理之后再做分析。

二.内部结构

1.开发环境设置

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

之所以一开始就设置环境变量是因为，CI框架中很多组件都依赖于ENVIRONMENT的配置，如system

localhost:system root# grep ENVIRONMENT ./* -R
./core/CodeIgniter.php:    if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/constants.php'))
./core/CodeIgniter.php:        require_once(APPPATH.'config/'.ENVIRONMENT.'/constants.php');
./core/Common.php:            if (file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/config.php'))
./core/Common.php:            if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/mimes.php'))
./core/Common.php:                $_mimes = include(APPPATH.'config/'.ENVIRONMENT.'/mimes.php');
./core/Config.php:            foreach (array($file, ENVIRONMENT.DIRECTORY_SEPARATOR.$file) as $location)
./core/Hooks.php:        if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
./core/Hooks.php:            include(APPPATH.'config/'.ENVIRONMENT.'/hooks.php');
./core/Loader.php:                    if (file_exists($path.'config/'.ENVIRONMENT.'/'.strtolower($class).'.php'))
./core/Loader.php:                        include($path.'config/'.ENVIRONMENT.'/'.strtolower($class).'.php');
./core/Loader.php:                    elseif (file_exists($path.'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php'))
./core/Loader.php:                        include($path.'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php');
./core/Loader.php:        if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/autoload.php'))
./core/Loader.php:            include(APPPATH.'config/'.ENVIRONMENT.'/autoload.php');
./core/Router.php:        if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/routes.php'))
./core/Router.php:            include(APPPATH.'config/'.ENVIRONMENT.'/routes.php');
./database/DB.php:        if ( ! file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/database.php')
./database/DB.php:                    if (file_exists($file_path = $path.'config/'.ENVIRONMENT.'/database.php'))
./helpers/html_helper.php:            if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/doctypes.php'))
./helpers/html_helper.php:                include(APPPATH.'config/'.ENVIRONMENT.'/doctypes.php');
./helpers/smiley_helper.php:            if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/smileys.php'))
./helpers/smiley_helper.php:                include(APPPATH.'config/'.ENVIRONMENT.'/smileys.php');
./helpers/text_helper.php:            if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/foreign_chars.php'))
./helpers/text_helper.php:                include(APPPATH.'config/'.ENVIRONMENT.'/foreign_chars.php');
./libraries/User_agent.php:        if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/user_agents.php'))
./libraries/User_agent.php:            include(APPPATH.'config/'.ENVIRONMENT.'/user_agents.php');

这样就可以在不改变代码的情况下，方便根据环境做一些特殊的不同的配置，如数据库配置文件的加载：

if (file_exists($file_path = $path.'config/'.ENVIRONMENT.'/database.php'))
{
    include($file_path);
}
elseif (file_exists($file_path = $path.'config/database.php'))
{
    include($file_path);
}

2. 定义常量信息

$system_path = 'system';
$application_folder = 'application';
$view_folder = '';

// 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));

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

其中省略了对$system_path、$application_folder、$view_folder的验证和正对不同系统的处理如对$view_folder的处理：

// 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
    }

还有这样一段代码，首先，STDIN、STDOUT、STDERR是PHP以 CLI（Command Line Interface）模式运行而定义的三个常量，这三个常量类似于Shell的stdin,stdout,stderr,分别是PHP CLI模式下的标准输入、标准输出和标准错误流。也就是说，这三行代码是为了保证命令行模式下，CI框架可以正常运行。关于PHP CLI的更多细节可以参考：http://www.php-cli.com/

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

3.引入CI的 boostrap file --CodeIgniter.php，这个文件是之后的处理流程关键

require_once BASEPATH.'core/CodeIgniter.php';