订单结算submit_order.php（扣库存,扣账号金额）

<?php
    header("content-type: text/html; charset=utf-8");
    session_start();
    include_once("db.class.php");

    //测试数据,初始化金额为1000.
    // $_session['account'] = 1000;


    // 获得订单总额
    $order_money = $_get['total'];

    //获得账户余额
    $uid = $_session['uid'];
    // var_dump($uid);
    $sql = "select account from tb_user where id = $uid";
    $account = $dao->getone($sql);

    if($account>=$order_money){
        $order = $_session['order'] ;
        // 从数据库中减掉库存
        
        foreach($order as $item){
            $kucun = $item['num'];
            $id = $item['goods_id'];
            $sql = "update tb_goods set number =number-{$kucun} where id ={$id}";
            // var_dump($sql);
            // die();
            $dao->exec($sql);
        }
        
        
            //减去金额    
        $account-=$order_money;
        $sql = "update tb_user set account = {$account} where id = $uid";
        $dao->exec($sql);
        echo "下单成功！账户余额：￥$account";
        $_session['order'] = '';
    }else{
        echo "下单失败！订单所需金额:$order_money,账户余额：￥$account,余额不足!!";
        
    }
    
    
?>

业务逻辑

下单之后用户要进行判断库存，和用户金额是否充足。

充足的话进行修改数据库的库存和余额。