Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
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 . $data);
    }
private function writeRichTextString($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);
    }
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