【PHP】CI框架原始碼DB.php（資料庫類）

CI框架很少有關於資料庫方面的原始碼分析，於是本人將資料庫類的DB.php的一些見解貼出來，希望對大家有幫助：
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defined('BASEPATH') OR exit('No direct script access allowed');

/**
 *初始化資料庫
 * Initialize the database
 *
 * @category    Database
 * @author    EllisLab Dev Team
 * @link    /user_guide/database/
 *
 * @param     string|string[]    $params
 * @param     bool        $query_builder_override
 *                Determines if query builder should be used or not
 */
function &DB($params = '', $query_builder_override = NULL)
{
    // Load the DB config file if a DSN string wasn't passed
    //如果DSN沒透過，則load 配置檔案
    if (is_string($params) && strpos($params, '://') === FALSE)
    {
        // Is the config file in the environment folder?
        if ( ! file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/database.php')
            && ! file_exists($file_path = APPPATH.'config/database.php'))
        {
            show_error('The configuration file database.php does not exist.');
        }
        //載入配置檔案
        include($file_path);

        // Make packages contain database config files,
        // given that the controller instance already exists
        //檢視包中是否時候有config/database.php這檔案，如果有，則載入
        if (class_exists('CI_Controller', FALSE))
        {
            //載入包，格式在APPPATH的config目錄中定義，格式如下：
            //$autoload['packages'] = array(APPPATH.'third_party', '/usr/local/shared');
            //get_package_paths()為CI_Loader::get_package_paths()
            foreach (get_instance()->load->get_package_paths() as $path)
            {
                if ($path !== APPPATH)
                {
                    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);
                    }
                }
            }
        }
        //$db為config/database.php中的配置變數
        if ( ! isset($db) OR count($db) === 0)
        {
            show_error('No database connection settings were found in the database config file.');
        }

        if ($params !== '')
        {
            $active_group = $params;
        }
        //$active_group沒有設定，則說明沒有資料庫連線
        if ( ! isset($active_group))
        {
            show_error('You have not specified a database connection group via $active_group in your config/database.php file.');
        }
        //$db[$active_group]為空，說明指定一個無效的資料庫連線組，預設為$db['default']
        elseif ( ! isset($db[$active_group]))
        {
            show_error('You have specified an invalid database connection group ('.$active_group.') in your config/database.php file.');
        }
        //引數為配置檔案中指定的陣列，預設為$db['default']
        $params = $db[$active_group];
    }
    elseif (is_string($params))
    {
        /**
         * Parse the URL from the DSN string
         * Database settings can be passed as discreet
         * parameters or as a data source name in the first
         * parameter. DSNs must have this prototype:
         * $dsn = 'driver://username:password@hostname/database';
         * 完整是這樣
         * driver://username:password@hostname/database?option1=value1&option2=value2..
         */
        if (($dsn = @parse_url($params)) === FALSE)
        {
            show_error('Invalid DB Connection String');
        }

        $params = array(
            'dbdriver'    => $dsn['scheme'],
            'hostname'    => isset($dsn['host']) ? rawurldecode($dsn['host']) : '',
            'port'        => isset($dsn['port']) ? rawurldecode($dsn['port']) : '',
            'username'    => isset($dsn['user']) ? rawurldecode($dsn['user']) : '',
            'password'    => isset($dsn['pass']) ? rawurldecode($dsn['pass']) : '',
            'database'    => isset($dsn['path']) ? rawurldecode(substr($dsn['path'], 1)) : ''
        );

        // Were additional config items set?
        // 是否有附加的配置項
        if (isset($dsn['query']))
        {
            //將dsn的查詢string解析到陣列$extra
            parse_str($dsn['query'], $extra);

            foreach ($extra as $key => $val)
            {
                if (is_string($val) && in_array(strtoupper($val), array('TRUE', 'FALSE', 'NULL')))
                {
                    $val = var_export($val, TRUE);
                }

                $params[$key] = $val;
            }
        }
    }

    // No DB specified yet? Beat them senseless...
    //db沒有選擇，show error
    if (empty($params['dbdriver']))
    {
        show_error('You have not selected a database type to connect to.');
    }

    // Load the DB classes. Note: Since the query builder class is optional
    // we need to dynamically create a class that extends proper parent class
    // based on whether we're using the query builder class or not.
    if ($query_builder_override !== NULL)
    {
        $query_builder = $query_builder_override;
    }
    // Backwards compatibility work-around for keeping the
    // $active_record config variable working. Should be
    // removed in v3.1
    //$active_record
    //沒有定義查詢構造器並且$active_record被設定
    //$active_record=TRUE,可以放入配置檔案中(database.php)
    //所謂的查詢構造器$user = DB::table('users')->where('name', 'John')->first();以這種方式進行訪問
    elseif ( ! isset($query_builder) && isset($active_record))
    {
        $query_builder = $active_record;
    }
    //包含CI_DB_driver類（抽象類）
    require_once(BASEPATH.'database/DB_driver.php');

    if ( ! isset($query_builder) OR $query_builder === TRUE)
    {
        //包含CI_DB_query_builder類
        require_once(BASEPATH.'database/DB_query_builder.php');
        if ( ! class_exists('CI_DB', FALSE))
        {
            /**
            *預設和$query_builder===TRUE時，繼承CI_DB_query_builder { }，否則繼承CI_DB_driver { }
            * CI_DB
             *
             *扮演CI_DB_driver和CI_DB_query_builder的別名
             * Acts as an alias for both CI_DB_driver and CI_DB_query_builder.
             *
             *
             * @see    CI_DB_query_builder
             * @see    CI_DB_driver
             */
            class CI_DB extends CI_DB_query_builder { }
        }
    }
    elseif ( ! class_exists('CI_DB', FALSE))
    {
        /**
          * @ignore
         */
        class CI_DB extends CI_DB_driver { }
    }

    // Load the DB driver
    $driver_file = BASEPATH.'database/drivers/'.$params['dbdriver'].'/'.$params['dbdriver'].'_driver.php';

    //不存在驅動檔案，則報錯
    file_exists($driver_file) OR show_error('Invalid DB driver');
    require_once($driver_file);

    // Instantiate the DB adapter
    //例項化DB介面卡，如果是mysql，$driver為CI_DB_mysql_driver ，繼承自上面的CI_DB
    //繼承關係:'CI_DB_'.$params['dbdriver'].'_driver' extends CI_DB extends CI_DB_driver(或者CI_DB_query_builders)
    $driver = 'CI_DB_'.$params['dbdriver'].'_driver';
    $DB = new $driver($params);

    // Check for a subdriver
    //檢視是否有子驅動
    if ( ! empty($DB->subdriver))
    {
        $driver_file = BASEPATH.'database/drivers/'.$DB->dbdriver.'/subdrivers/'.$DB->dbdriver.'_'.$DB->subdriver.'_driver.php';

        if (file_exists($driver_file))
        {
            require_once($driver_file);
            $driver = 'CI_DB_'.$DB->dbdriver.'_'.$DB->subdriver.'_driver';
            //例項化子驅動
            $DB = new $driver($params);
        }
    }
    //初始化資料庫設定，包含建立連線以及設定客戶端字符集
    $DB->initialize();
    //返回DB例項
    return $DB;
}