SUM grupo de filas por nombre usando pdo

Intenta lo siguiente:

Solo tenga en cuenta que he arreglado sql para usted.

<?php 
    $sql = "SELECT itemname, SUM(quantity) AS quantity from tblorders GROUP BY itemname";
    $query = $dbh -> prepare($sql);
    $query->execute();
    $results=$query->fetchAll(PDO::FETCH_OBJ);
    $cnt=1;
    if($query->rowCount() > 0) {
        foreach($results as $result) { ?>  

        <tr>
            <td> <?php echo htmlentities($cnt);?></td>
            <td><?php echo htmlentities($result->itemname);?></td>
            <td><?php echo htmlentities ($result->quantity);?></td>
        </tr>

<?php 
            $cnt++;
        } 
    }
?>