问题是：

我试图从数据库users中的可变现金金额中减去0.05，我用ajax调用这个文件，但是什么都没有发生。要解决此问题，我在浏览器中打开了文件，出现以下错误：

错误：

sql语法有错误；请查看与mysql服务器版本对应的手册，以获取在第1行的“”附近使用的正确语法

代码：

PHP：

<?php
session_start();

$servername = "localhost";
$username = "myUser";
$password = "myPass";
$dbname = "myDBname";
$cash_amount = $_SESSION['cash_amount'];

// Create connection

$userid = $_SESSION['id'];

// You must enter the user's id here. /\

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

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

// Fetch the existing value of the cash_amount against that particular user here. You can use the SELECT cash_amount from users where userid = $userid
$_SESSION['cash_amount'] -= 0.05;
$newAmount = $cash_amount - 0.05;

$sql = "UPDATE users SET cash_amount = $newAmount WHERE id = $userid";
$result = $conn->query($sql);

if($result)
{
   echo "5 cents have been subtracted!";
}
else
{
   echo mysqli_error($conn);
   session_start();
   session_unset();
   session_destroy();
}

$conn->close();
?>

javascript/ajax：

function countdownEnded() {
    //make serverscreen dissapear
        document.getElementById('serverScreenWrapper').style.display = 'none';
        document.getElementById('serverScreenWrapper').style.opacity = '0';
        document.getElementById("cashOutNumTwo").style.right = '150%';
        document.getElementById("cashOutNumOne").style.right = '150%';
//start Timer
        setInterval(gameTimer.update, 1000);
//make player move again
        socket.emit('4');
        socket.emit('6');
//make game appear
        document.getElementById('gameAreaWrapper').style.opacity = 1;
//play sound
        document.getElementById('spawn_cell').play();
//cut 5 cents from account - php function
        $.ajax({
        type: "POST",
        url: 'http://cashballz.net/game/5game/subtract5.php',
        data: { },
        success: function (data) {
            alert(data);
        }
    });
}

我的数据库：

我的表名为users，它位于db casu帐户的内部。
格式如下：
id |名字|姓氏|电子邮件|密码|现金|金额| 4介于|哈希|活动

结论：

我很困惑为什么我的php代码不起作用，我已经尝试过搜索一个修复程序，我发现了“sql注入”这个词，但仍然没有找到错误。我在js方面是高级的，但对php来说是初学者，所以请耐心听我说。谢谢！