Isih hasil mengikut series_id supaya semua produk dengan nilai yang sama akan bersama.

$stmt = $pdo->prepare("SELECT series_id, product_name
                       FROM yourTable
                       ORDER BY series_id");
$stmt->execute();

Kemudian apabila menunjukkan keputusan, tunjukkan tajuk siri dan mulakan yang baharu apabila ia berubah

$last_series = null;
echo "<ul>\n";
while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    if ($row['series_id'] != $last_series) {
        if ($last_series) {
            echo "</ul></li>\n";
        }
        echo "<li>" . $row['series_id'] . " Series Product\n";
        echo "<ul>\n";
        $last_series = $row['series_id'];
    }
    echo "<li>" . $row['product_name'] . "</li>\n";
}
if ($last_series) {
    echo "</li>\n";
}
echo "</ul>\n";