code: private function writeRichTextString($row, $col, $str, $xfIndex, $arrcRun)
      {
          $record = 0x00FD; // Record identifier
          $length = 0x000A; // Bytes to follow
          $str = StringHelper::UTF8toBIFF8UnicodeShort($str, $arrcRun);
  
          // 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 LABELSST record or a LABEL record. Which one depends on BIFF version
  It differs from writeString by the writing of rich text strings.
  
  @param int $row Row index (0-based)
  @param int $col Column index (0-based)
  @param string $str The string
  @param int $xfIndex The XF format index for the cell
  @param array $arrcRun Index to Font record and characters beginning