PHPExcel: añadir filas nuevas a un fichero XLS ya existente

require_once './librerias/PHPExcel/IOFactory.php';
$objPHPExcel = PHPExcel_IOFactory::load("./hist_form.xlsx");
$highestColumm = $objPHPExcel->setActiveSheetIndex(0)->getHighestColumn();
$highestRow = $objPHPExcel->setActiveSheetIndex(0)->getHighestRow();
$highestRow++;
$objPHPExcel->setActiveSheetIndex(0);
$fh_actual = date('d-m-Y H:i:s');
$objPHPExcel->getActiveSheet()->setCellValue('A'.$highestRow, $fh_actual);
$objPHPExcel->getActiveSheet()->setCellValue('B'.$highestRow, $_POST["cif"]);
$objPHPExcel->getActiveSheet()->setCellValue('C'.$highestRow, $_POST["nombre"]);
$objPHPExcel->getActiveSheet()->setCellValue('D'.$highestRow, $_POST["apellidos"]);
$objPHPExcel->getActiveSheet()->setCellValue('E'.$highestRow, $_POST["email"]);
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter->save('hist_form.xlsx', __FILE__);