wordpress全局变量$wpdb在哪里进行初始化声明

define('WP_USE_THEMES', true);
/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wp-blog-header.php' );

if ( !isset($wp_did_header) ) {
$wp_did_header = true;
require_once( dirname(__FILE__) . '/wp-load.php' );
wp();
require_once( ABSPATH . WPINC . '/template-loader.php' );
}

if ( file_exists( ABSPATH . 'wp-config.php') ) {
/** The config file resides in ABSPATH */
require_once( ABSPATH . 'wp-config.php' );
} elseif ( file_exists( dirname(ABSPATH) . '/wp-config.php' ) && ! file_exists( dirname(ABSPATH) . '/wp-settings.php' ) ) {
/** The config file resides one level above ABSPATH but is not part of another install */
require_once( dirname(ABSPATH) . '/wp-config.php' );
} else {
……
}

/* 好了！请不要再继续编辑。请保存本文件。使用愉快！ */
/** WordPress目录的绝对路径。 */
if ( !defined('ABSPATH') )
define('ABSPATH', dirname(__FILE__) . '/');
/** 设置WordPress变量和包含文件。 */
require_once(ABSPATH . 'wp-settings.php');

define( 'WPINC', 'wp-includes' );
// Include files required for initialization.
require( ABSPATH . WPINC . '/load.php' );
require( ABSPATH . WPINC . '/default-constants.php' );
require( ABSPATH . WPINC . '/version.php' );

function require_wp_db() {
global $wpdb;
require_once( ABSPATH . WPINC . '/wp-db.php' );
if ( file_exists( WP_CONTENT_DIR . '/db.php' ) )
require_once( WP_CONTENT_DIR . '/db.php' );
if ( isset( $wpdb ) )
return;
$wpdb = new wpdb( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );
}

// Include the wpdb class and, if present, a db.php database drop-in.
require_wp_db();