$objPHPExcel = PHPExcel_IOFactory::load('./test.xlsx');
$comment = $objPHPExcel->getActiveSheet()->getComment('E11');
$commentText = new PHPExcel_RichText;
$commentText->createText("testtesttest");
$comment->setText($commentText);
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, "Excel2007");
$objWriter->save('./test.xlsx');