code: public function writeFont()
      {
          $font_outline = 0;
          $font_shadow = 0;
  
          $icv = $this->colorIndex; // Index to color palette
          if ($this->font->getSuperscript()) {
              $sss = 1;
          } elseif ($this->font->getSubscript()) {
              $sss = 2;
          } else {
              $sss = 0;
          }
          $bFamily = 0; // Font family
          $bCharSet = \PhpOffice\PhpSpreadsheet\Shared\Font::getCharsetFromFontName($this->font->getName()); // Character set
  
          $record = 0x31; // Record identifier
          $reserved = 0x00; // Reserved
          $grbit = 0x00; // Font attributes
          if ($this->font->getItalic()) {
              $grbit |= 0x02;
          }
          if ($this->font->getStrikethrough()) {
              $grbit |= 0x08;
          }
          if ($font_outline) {
              $grbit |= 0x10;
          }
          if ($font_shadow) {
              $grbit |= 0x20;
          }
  
          $data = pack(
              'vvvvvCCCC',
              // Fontsize (in twips)
              $this->font->getSize() * 20,
              $grbit,
              // Colour
              $icv,
              // Font weight
              self::mapBold($this->font->getBold()),
              // Superscript/Subscript
              $sss,
              self::mapUnderline($this->font->getUnderline()),
              $bFamily,
              $bCharSet,
              $reserved
          );
          $data .= StringHelper::UTF8toBIFF8UnicodeShort($this->font->getName());
  
          $length = strlen($data);
          $header = pack('vv', $record, $length);
  
          return $header . $data;
      }
docs: Get font record data.
  
  @return string