conbinar columnas en phpexcel

<?php

include "PHPExcel.php";
$excel=new PHPExcel();
$excel->setActiveSheetIndex(0);
$excel->getActiveSheet()->setCellValue('A1', 'Hola Mundo');
$excel->getActiveSheet()->setCellValue('B1', 'Hola Mundo');
$excel->getActiveSheet()->setCellValue('C1', 'Hola Mundo');
$excel->getActiveSheet()->setCellValue('D1', 'D');
$excel->getActiveSheet()->setCellValue('E1', 'E');
$excel->getActiveSheet()->setCellValue('F1', 'F');
$excel->getActiveSheet()->setCellValue('G1', 'G');
$excel->getActiveSheet()->setCellValue('H1', 'H');
$excel->getActiveSheet()->setCellValue('I1', 'I');
$excel->getActiveSheet()->setCell