code: private function readSheetRangeByRefIndex($index)
      {
          if (isset($this->ref[$index])) {
              $type = $this->externalBooks[$this->ref[$index]['externalBookIndex']]['type'];
  
              switch ($type) {
                  case 'internal':
                      // check if we have a deleted 3d reference
                      if ($this->ref[$index]['firstSheetIndex'] == 0xFFFF or $this->ref[$index]['lastSheetIndex'] == 0xFFFF) {
                          throw new Exception('Deleted sheet reference');
                      }
  
                      // we have normal sheet range (collapsed or uncollapsed)
                      $firstSheetName = $this->sheets[$this->ref[$index]['firstSheetIndex']]['name'];
                      $lastSheetName = $this->sheets[$this->ref[$index]['lastSheetIndex']]['name'];
  
                      if ($firstSheetName == $lastSheetName) {
                          // collapsed sheet range
                          $sheetRange = $firstSheetName;
                      } else {
                          $sheetRange = "$firstSheetName:$lastSheetName";
                      }
  
                      // escape the single-quotes
                      $sheetRange = str_replace("'", "''", $sheetRange);
  
                      // if there are special characters, we need to enclose the range in single-quotes
                      // todo: check if we have identified the whole set of special characters
                      // it seems that the following characters are not accepted for sheet names
                      // and we may assume that they are not present: []*/:\?
                      if (preg_match("/[ !\"@#£$%&{()}<>=+'|^,;-]/u", $sheetRange)) {
                          $sheetRange = "'$sheetRange'";
                      }
  
                      return $sheetRange;
  
                      break;
                  default:
                      // TODO: external sheet support
                      throw new Exception('Xls reader only supports internal sheets in formulas');
  
                      break;
              }
          }
  
          return false;
      }
docs: Get a sheet range like Sheet1:Sheet3 from REF index
  Note: If there is only one sheet in the range, one gets e.g Sheet1
  It can also happen that the REF structure uses the -1 (FFFF) code to indicate deleted sheets,
  in which case an Exception is thrown.
  
  @param int $index
  
  @throws Exception
  
  @return false|string