private function generateReferenceNumber($prefix = null, $length = 8)
  {
    $az = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZMULTIPAY';
    $azr = rand(0, 51);
    $azs = substr($az, $azr, 10);
    $stamp = hash('sha256', time());
    $mt = hash('sha256', mt_rand(5, 20));
    $alpha = hash('sha256', $azs);
    $hash = str_shuffle($stamp . $mt . $alpha);
    $code = strtoupper(substr($hash, $azr, $length));

    if (!is_null($prefix) && !empty(trim($prefix))) {
      $code = $prefix . $code;
    }

    return $code;
  }