Codificar cabeçalhos para arquivo do Excel usando PHP enquanto consulta SQL dinamicamente

$objPHPExcel = new PHPExcel();
$objPHPExcel->setActiveSheetIndex(0)
            ->setCellValue('A1', 'Hello')
            ->setCellValue('A2', 'world!');

$col = 1; 
while($row_data = mysql_fetch_assoc($result)) {
    $row = 1;
    foreach($row_data as $value) {
        $objPHPExcel->getActiveSheet()->setCellValueByColumnAndRow($col, $row, $value);
        $row++;
    }
    $col++;
}