Section 4.6.34 of the developer documentation for merging cells:
Section 4.6.20 for setting alignment:
$objPHPExcel->getActiveSheet()->mergeCells('A18:E22');
$objPHPExcel->getActiveSheet()->getStyle('A18:E22')
->getAlignment()
->setHorizontal(PHPExcel_Style_Alignment::HORIZONTAL_CENTER);