private function format_s(&$str) {
    if (strstr($str, '%s'))
      $str = str_replace('%s', sprintf('%02d', $this->sec), $str);
  }
private function format_s(&$str) {
    if (strstr($str, '%s'))
      $str = str_replace('%s', sprintf('%02d', $this->sec), $str);
  }
Seconds with leading zeros
@param type $str