public function daysOfWeek($daysOfWeek)
  {
    // Check for empty, string or no value
    if (empty($daysOfWeek) || is_string($daysOfWeek) === false || strlen($daysOfWeek) < 3) {

      // Return with a null parse
      return null;
    }

    // Separate days of the week by comma
    $daysOfWeek = explode(',', $daysOfWeek);

    // Allowed values
    $allowedValues = ['mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun'];

    // Make sure each day of week
    foreach ($daysOfWeek as $dayOfWeek) {

      // Is an allowed value
      if (in_array($dayOfWeek, $allowedValues) === false) {

        // Otherwise return with a null parse
        return null;
      }
    }

    // Return days of the week
    return $daysOfWeek;
  }