PHP存款/取款，不能扣除

Question

晚上好

我想问一下这个问题。我正在尝试编写一个程序，当我点击转移时，它将从余额中扣除金额并使用mysql数据库中的新余额进行更新。

我的代码看起来像这样

修改         

$myotp = $_POST['otp'];
if(OTPPASS==$myotp)
{
    $amt = $POST['amt'];
    require 'config/config.php';
    $con = mysqli_connect($hostname, $user, $pass, $db) or die ('Cannot connect');
    $sql = "select openBal from accInfo where userID ='".$_SESSION['userID']."'";
    $result = mysqli_query($con,$sql);
    $row = mysqli_fetch_array($result);
    $balance = $row['openBal'];
    $account = $row['accountNo'];
    $sql2 = "update accInfo set openBal='".$balance."'-$amt here accountNo='".$account."'";
    mysqli_query($con,$sql2);

    echo "Transfer Complete";
    mysqli_close($con);

    header('Location: success.html');
}else{
        echo ("<SCRIPT LANGUAGE='JavaScript'>
    window.alert('Transaction Failed')
    window.location.href='transfers.php';
    </SCRIPT>");    
}
?>

问题是它没有从数据库行中扣除余额，$row['openBal']我需要对此进行进一步解释，寻找互联网并找不到好文章。

我开始考虑可能是我设置sql文件的方式..