PDO FETCH返回列名而不是值

Question

$forData = "SELECT item, GROUP_CONCAT(? ORDER BY ?) AS chartData 
FROM old_record_sales
WHERE (item = ? OR ? IS NULL)
GROUP BY item";

$stmt = $dbcon->prepare($forData);
$params = [
    $type,
    $sort,
    $flwr,
    $flwr
];

$stmt->execute($params);

while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    extract($row);
    $data[] = $row['chartData'];
    $dataNames[] = $row['item'];
}

每当我回显$data[]的值时，它都会显示列名，而不是我指定的列中的值。如何获得$data[] = $row['chartData'];将该列的值插入$data中？

如果我这样写就可以了：

GROUP_CONCAT($type ORDER BY ?)

预先感谢您的帮助。