code: private function readHeader()
      {
          $length = self::getUInt2d($this->data, $this->pos + 2);
          $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);
  
          // move stream pointer to next record
          $this->pos += 4 + $length;
  
          if (!$this->readDataOnly) {
              // offset: 0; size: var
              // realized that $recordData can be empty even when record exists
              if ($recordData) {
                  if ($this->version == self::XLS_BIFF8) {
                      $string = self::readUnicodeStringLong($recordData);
                  } else {
                      $string = $this->readByteStringShort($recordData);
                  }
  
                  $this->phpSheet->getHeaderFooter()->setOddHeader($string['value']);
                  $this->phpSheet->getHeaderFooter()->setEvenHeader($string['value']);
              }
          }
      }
docs: Read HEADER record.