code: public function mergeCells($pRange)
      {
          // Uppercase coordinate
          $pRange = strtoupper($pRange);
  
          if (strpos($pRange, ':') !== false) {
              $this->mergeCells[$pRange] = $pRange;
  
              // make sure cells are created
  
              // get the cells in the range
              $aReferences = Coordinate::extractAllCellReferencesInRange($pRange);
  
              // create upper left cell if it does not already exist
              $upperLeft = $aReferences[0];
              if (!$this->cellExists($upperLeft)) {
                  $this->getCell($upperLeft)->setValueExplicit(null, DataType::TYPE_NULL);
              }
  
              // Blank out the rest of the cells in the range (if they exist)
              $count = count($aReferences);
              for ($i = 1; $i < $count; ++$i) {
                  if ($this->cellExists($aReferences[$i])) {
                      $this->getCell($aReferences[$i])->setValueExplicit(null, DataType::TYPE_NULL);
                  }
              }
          } else {
              throw new Exception('Merge must be set on a range of cells.');
          }
  
          return $this;
      }
docs: Set merge on a cell range.
  
  @param string $pRange Cell range (e.g. A1:E1)
  
  @throws Exception
  
  @return Worksheet