code: private function writeNumberFormat($format, $ifmt)
      {
          $record = 0x041E; // Record identifier
  
          $numberFormatString = StringHelper::UTF8toBIFF8UnicodeLong($format);
          $length = 2 + strlen($numberFormatString); // Number of bytes to follow
  
          $header = pack('vv', $record, $length);
          $data = pack('v', $ifmt) . $numberFormatString;
          $this->append($header . $data);
      }
docs: Writes Excel FORMAT record for non "built-in" numerical formats.
  
  @param string $format Custom format string
  @param int $ifmt Format index code