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++;    }}