code: private function writeCFHeader()
      {
          $record = 0x01B0; // Record identifier
          $length = 0x0016; // Bytes to follow
  
          $numColumnMin = null;
          $numColumnMax = null;
          $numRowMin = null;
          $numRowMax = null;
          $arrConditional = [];
          foreach ($this->phpSheet->getConditionalStylesCollection() as $cellCoordinate => $conditionalStyles) {
              foreach ($conditionalStyles as $conditional) {
                  if ($conditional->getConditionType() == Conditional::CONDITION_EXPRESSION
                          || $conditional->getConditionType() == Conditional::CONDITION_CELLIS) {
                      if (!in_array($conditional->getHashCode(), $arrConditional)) {
                          $arrConditional[] = $conditional->getHashCode();
                      }
                      // Cells
                      $arrCoord = Coordinate::coordinateFromString($cellCoordinate);
                      if (!is_numeric($arrCoord[0])) {
                          $arrCoord[0] = Coordinate::columnIndexFromString($arrCoord[0]);
                      }
                      if ($numColumnMin === null || ($numColumnMin > $arrCoord[0])) {
                          $numColumnMin = $arrCoord[0];
                      }
                      if ($numColumnMax === null || ($numColumnMax < $arrCoord[0])) {
                          $numColumnMax = $arrCoord[0];
                      }
                      if ($numRowMin === null || ($numRowMin > $arrCoord[1])) {
                          $numRowMin = $arrCoord[1];
                      }
                      if ($numRowMax === null || ($numRowMax < $arrCoord[1])) {
                          $numRowMax = $arrCoord[1];
                      }
                  }
              }
          }
          $needRedraw = 1;
          $cellRange = pack('vvvv', $numRowMin - 1, $numRowMax - 1, $numColumnMin - 1, $numColumnMax - 1);
  
          $header = pack('vv', $record, $length);
          $data = pack('vv', count($arrConditional), $needRedraw);
          $data .= $cellRange;
          $data .= pack('v', 0x0001);
          $data .= $cellRange;
          $this->append($header . $data);
      }
docs: Write CFHeader record.