code: private function writeLabelSst($row, $col, $str, $xfIndex)
      {
          $record = 0x00FD; // Record identifier
          $length = 0x000A; // Bytes to follow
  
          $str = StringHelper::UTF8toBIFF8UnicodeLong($str);
  
          // check if string is already present
          if (!isset($this->stringTable[$str])) {
              $this->stringTable[$str] = $this->stringUnique++;
          }
          ++$this->stringTotal;
  
          $header = pack('vv', $record, $length);
          $data = pack('vvvV', $row, $col, $xfIndex, $this->stringTable[$str]);
          $this->append($header . $data);
      }
docs: Write a string to the specified row and column (zero indexed).
  This is the BIFF8 version (no 255 chars limit).
  $format is optional.
  
  @param int $row Zero indexed row
  @param int $col Zero indexed column
  @param string $str The string to write
  @param mixed $xfIndex The XF format index for the cell