我想从字段(B)币值中减去字段(A) - coin_charge值，并将结果存储在字段(B)币中 - I want to subtract field (A) - coin_charge value from field (B) coin value and store the result in field (B) coin

I want to subtract field (A) - coin_charge value from field (B) coin value and store the result in field (B) coin

首先，我在MySQL数据库字段中得到了两个字段，COIN和字段COIN_CHARGE这里是屏幕截图弹出Screnshot,我希望系统检查用户是否单击YES，即试图顶峰并继续进行，让系统检查他的数据库数据库屏幕截图并开始这个参数，(如果字段-coin-小于字段-coin_charge-)应该弹出一个窗口说，资金不足，无法继续充值您的帐户，否则，如果硬币字段大于coin_charge字段，让系统从硬币中减去coin_charge，也就是说，如果(coin = 100)和(coin_charge =50)硬币将变成50

----------------- 弹出消息 ------------------------------

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--------------------------- 计算代码 ---------------------------

<?php
    $message_query = mysql_query("select * from student ")or die(mysql_error());
?>
<?php
    while ($row = mysql_fetch_assoc($results)) {
        echo $row['coin'] = $row['coin'] - $row['coin_charge'];
    }
?>

$mysqli = new mysqli("localhost", "username", "password", "dbname");
$userid = $_POST["userid"];
$stmt = $mysqli->prepare('SELECT * FROM student WHERE id = ?');
$stmt->bind_param('i', $userid);
$stmt->execute();
$result = $stmt->get_result();
$row = $result->fetch_assoc();
$stmt->close();
$total = $row['coin'] - $row['coin_charge'];
if($total > 0){
    $stmt = $mysqli->prepare("update student set coin = ? where id = ?");
    $stmt->bind_param("ii", $total, $userid);
    $stmt->execute();
    $stmt->close();
    echo $total;
}else{
    echo 'Insufficient Fund to proceed Top Up your account';exit;
}