Merging cells in Excel by rows and columns together using PHPExcel

There is one more method for cell merging

    /**
 * Set merge on a cell range by using numeric cell coordinates
 *
 * @param   int $pColumn1   Numeric column coordinate of the first cell
 * @param   int $pRow1      Numeric row coordinate of the first cell
 * @param   int $pColumn2   Numeric column coordinate of the last cell
 * @param   int $pRow2      Numeric row coordinate of the last cell
 * @throws  Exception
 * @return PHPExcel_Worksheet
 */
     public function mergeCellsByColumnAndRow($pColumn1 = 0, $pRow1 = 1, $pColumn2 = 0, $pRow2 = 1)

Merging simply requires a valid range of cells like A1:B2, so your

$sheet->mergeCells("G".($row_count+1).":I".($row_count+1));

should work without any problem.

Can you please experiment with a simple test case to prove that this is causing you a problem, and not something else in your script

EDIT

After rereading your question: Your problem may be that you're trying to merge cells that are already part of a merge range, rather than merging each row, then trying to merge by column, try merging the full rangein one go.

$sheet->mergeCells("G".($row_count+1).":I".($row_count+4));