code: private function writeString($row, $col, $str, $xfIndex)
      {
          $this->writeLabelSst($row, $col, $str, $xfIndex);
      }
docs: Write a LABELSST record or a LABEL record. Which one depends on BIFF version.
  
  @param int $row Row index (0-based)
  @param int $col Column index (0-based)
  @param string $str The string
  @param int $xfIndex Index to XF record