static function setCredentials($sid, $privateKey, $mode = '')
 {
     self::$sid = $sid;
     self::$privateKey = $privateKey;
     if ($mode == 'sandbox') {
         self::$apiUrl = 'https://sandbox.2checkout.com/checkout/api/1/' . $sid . '/rs/authService';
     } else {
         self::$apiUrl = 'https://www.2checkout.com/checkout/api/1/' . $sid . '/rs/authService';
     }
 }