php更新函数不起作用

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// 创建连接
$conn = mysqli_connect($servername, $username, $password, $dbname);

// 检查连接
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

$sql = "UPDATE MyGuests SET lastname='Doe' WHERE id=1";
if (mysqli_query($conn, $sql)) {
    echo "Record updated successfully";
} else {
    echo "Error updating record: " . mysqli_error($conn);
}

try {
    $conn->beginTransaction();
    $sql = "UPDATE MyGuests SET lastname='Doe' WHERE id=1";
    $conn->exec($sql);
    $conn->commit();
    echo "Record updated successfully";
} catch (Exception $e) {
    $conn->rollback();
    echo "Error updating record: " . $e->getMessage();
}

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

try {
    // 创建PDO连接
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    // 准备SQL语句
    $sql = "UPDATE MyGuests SET lastname='Doe' WHERE id=1";

    // 执行SQL语句
    $stmt = $conn->prepare($sql);
    $stmt->execute();

    echo "Record updated successfully";
} catch(PDOException $e) {
    echo "Error updating record: " . $e->getMessage();
}

// 关闭连接
$conn = null;
?>