Unknown MySQL server host 'DB_SERVER' (11004)

[Rosalie Stratford]

I'm currently doing a php oop tutorial, with xamp installed and apache configured for virtual hosts.

I have a config.php page with the constants defined: DB_SERVER(set to 'localhost'), DB_USER, DB_PASS and DB_NAME, all set with username, pswd and db name accordingly.

My database.php page require_once the above and begins as follow:

class MySQLDatabase {

Private $connection;

function __construct() {
$this->open_connection();
}

public function open_connection() {
$this->connection = mysql_connect(DB_SERVER, DB_USER, DB_PASS);
if (!$this->connection) {
die("Database connection failed: " . mysql_error());
} else {
$db_select = mysql_select_db(DB_NAME, $this->connection);
if (!$db_select) {
die("Database selection failed: " . mysql_error());
}
}
}

The public function open_connection(), 2nd line beginning:$this->connection shows as a mysql_connection() error - Unknown MySQL server host "DB_SERVER"(11004).

Forgive my ignorance as I'm learning. Can anyone advise me please?

Many thanks.