带有来自 sql 数据库的值的表没有出现

Question

我写了这段代码，它应该用用户兑换的所有代金券和它们的价值制作一个表格，但它不起作用（根本没有出现，也没有显示任何错误）所以我很困惑我在这里做错了什么. 有人可以帮忙吗？

$dsn = 'mysql:host=127.0.0.1;dbname=user_db;charset=utf8';

$conn = new PDO($dsn, $username1, $password);

$conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

$stm_voucher = $conn->prepare("SELECT id FROM users WHERE username = :username");

$stm_voucher->bindParam(":username", $username, PDO::PARAM_INT);

$stm_voucher->execute();

while ($row = $stm_voucher->fetch(PDO::FETCH_ASSOC)) {

    $id = $row["id"];

    $stm_voucher2 = $conn->prepare("SELECT * FROM vouchers WHERE used_by = :id");

    $stm_voucher2->bindParam(":id", $id, PDO::PARAM_INT);

    $stm_voucher2->execute();

    $rank = 1;

    while ($row2 = $stm_voucher2->fetch(PDO::FETCH_ASSOC)) {

        echo "<tr>

            <td>{$rank}</td>

            <td>{$row2['voucher']}</td>

            <td>{$row2['value']}</td>

            <td>{$row2['used_on']}</td>

            </tr>";

        $rank++;

    }

}