找出这串代码的错误并给出正确答案<?php session_start(); $upd = $_GET['upd']; $id = $_GET['id']; $order = $_SESSION['cart']; foreach($order as $key => $value) { if($value['id'] == $id) { switch($upd) { case 0: if($value['num'] > 1) { $order[$key]['num'] -= 1; } else{ unset ($order[$key]); } break; case 1: $order[$key]['num'] += 1; break; default: } header("location:cart.php"); } if($upd == "") { $sum += $value['price'] * $value['num']; header("location:order.php"); } } $_SESSION['num'] = array_sum(array_column($order,"num")); $_SESSION['sum'] = $sum; $_SESSION['cart'] = $order; ?>

这段代码的错误在于变量 $sum 没有进行初始化。在 foreach 循环外部应该添加 $sum = 0; 进行初始化。正确的代码如下：

<?php 
session_start(); 
$upd = $_GET['upd']; 
$id = $_GET['id']; 
$order = $_SESSION['cart']; 
$sum = 0; // 初始化 $sum
foreach($order as $key => $value) {         
    if($value['id'] == $id) {                         
        switch($upd) {                                 
            case 0:                                 
                if($value['num'] > 1) {                                         
                    $order[$key]['num'] -= 1;                                 
                } else {                                 
                    unset ($order[$key]);                                 
                }                                 
                break;                                 
            case 1:                                 
                $order[$key]['num'] += 1;                                 
                break;                                 
            default:                         
        }                         
        header("location:cart.php");                 
    }                 
    if($upd == "") {                         
        $sum += $value['price'] * $value['num'];                         
        header("location:order.php");                 
    }         
}         
$_SESSION['num'] = array_sum(array_column($order,"num"));         
$_SESSION['sum'] = $sum;         
$_SESSION['cart'] = $order; 
?>