PHP怎么通过查询MySQL来处理用户充值

<fORM action="recharge.php" method="post">
    <label for="amount">Amount:</label>
    <input type="text" name="amount" id="amount">
    <button type="submit">Recharge</button>
</form>

<?php
//连接数据库
$servername = "localhost";
$username = "username";
$passWord = "password";
$dbname = "myDB";

$conn = new mysqli($servername, $username, $password, $dbname);

//检测连接
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

//处理表单数据
$amount = $_POST["amount"];

//将充值金额添加到数据库
$sql = "UPDATE users SET balance = balance + $amount WHERE id = 1";

if ($conn->query($sql) === TRUE) {
    echo "Recharge successful";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>