未定义常量DB_SERVER -假设'DB_SERVER'

我已经在本地机器上安装了WAMP服务器，我已经编写了php脚本将记录插入数据库，但我得到以下错误-

注意:使用未定义的常量DB_SERVER -假设'DB_SERVER'在C:'wamp'www'testrongervices'db_connect.php on line 32

    <?php 
    define('DB_USER', "root"); // db user
    define('DB_PASSWORD', ""); // db password
    define('DB_DATABASE', "test_db"); // database name
    define('DB_SERVER', "localhost"); // db server
    ?>
Here is my **db_connect.php code**
   <?php
/**
 * A class file to connect to database
 */
class DB_CONNECT 
{
    // constructor
    function __construct()
    {
        // connecting to database
        $this->connect();
    }
    // destructor
    function __destruct()
    {
        // closing db connection
        $this->close();
    }
    /**
     * Function to connect with database
     */
    function connect() 
    {
        // import database connection variables
        require_once __DIR__ . '/db_config.php';
        // Connecting to mysql database
        $con = mysql_connect(DB_SERVER, DB_USER, DB_PASSWORD) or die(mysql_error());
        // Selecing database
        $db = mysql_select_db(DB_DATABASE) or die(mysql_error()) or die(mysql_error());
        // returing connection cursor
        return $con;
    }
    /**
     * Function to close db connection
     */
    function close() 
    {
        // closing db connection
        mysql_close();
    }
}
?>

请帮. .谢谢你. . !