php - 防止列低于零和回显警告

Question

我有一个带有 character_gold 列的表。使用 character_gold 您可以训练力量（另一列）。但是即使 character_gold 为 0 也会继续训练，因此 character_gold 值变为负数：

if (isset($_POST['train_strength'])) {
    $trainstrength = "UPDATE user_character
        SET strength_trains = strength_trains + 1,
        trained_strength = trained_strength +1,
        character_gold = character_gold - $gold_to_next_strength
        WHERE ID = $currentUser";
    mysqli_query($con, $trainstrength);
    mysqli_close($con);

    header('Location: account.php');
}

我该如何防止这种情况并同时发出警报？

标签： phpmysql