PHP 購物車核心類別實作解析：Session 識別、資料庫操作與金額計算

前言

購物車是電商系統核心模組。本文解析一個經典的 PHP 購物車類別：使用 Session + Cookie 識別用戶，MySQL 持久化商品明細，提供增刪改查、金額計算、內容顯示等完整功能。

核心設計

• 識別機制：scid Cookie 存放 Session ID（MD5 唯一值），有效期 4 小時。
• 資料表結構：cart (id, session, product, quantity) + inventory (product, price, description)。
• 核心方法：檢查/新增/刪除/修改/清空/總價/明細/商品種類數/總數量。

完整類別代碼

<?php
// Session/Cookie 初始化（放在入口檔案或共用 include）
if (!isset($_COOKIE['scid']) && !isset($_SESSION['scid'])) {
    $session = md5(uniqid(rand(), true));
    setcookie('scid', $session, time() + 14400); // 4 小時
    $_SESSION['scid'] = $session;
} else {
    $session = $_COOKIE['scid'] ?? $_SESSION['scid'];
}

class Cart {
    private $table = 'cart';

    // 檢查商品是否已在購物車
    public function check_item($session, $product) {
        $sql = "SELECT quantity FROM {$this->table} WHERE session=? AND product=?";
        $stmt = $GLOBALS['db']->prepare($sql);
        $stmt->execute([$session, $product]);
        $row = $stmt->fetch(PDO::FETCH_ASSOC);
        return $row ? (int)$row['quantity'] : 0;
    }

    // 新增或增加數量
    public function add_item($session, $product, $quantity) {
        $qty = $this->check_item($session, $product);
        if ($qty == 0) {
            $sql = "INSERT INTO {$this->table} (session, product, quantity) VALUES (?, ?, ?)";
            $GLOBALS['db']->execute($sql, [$session, $product, $quantity]);
        } else {
            $quantity += $qty;
            $sql = "UPDATE {$this->table} SET quantity=? WHERE session=? AND product=?";
            $GLOBALS['db']->execute($sql, [$quantity, $session, $product]);
        }
    }

    // 刪除商品
    public function delete_item($session, $product) {
        $sql = "DELETE FROM {$this->table} WHERE session=? AND product=?";
        $GLOBALS['db']->execute($sql, [$session, $product]);
    }

    // 修改數量
    public function modify_quantity($session, $product, $quantity) {
        $sql = "UPDATE {$this->table} SET quantity=? WHERE session=? AND product=?";
        $GLOBALS['db']->execute($sql, [$quantity, $session, $product]);
    }

    // 清空購物車
    public function clear_cart($session) {
        $sql = "DELETE FROM {$this->table} WHERE session=?";
        $GLOBALS['db']->execute($sql, [$session]);
    }

    // 計算總金額
    public function cart_total($session) {
        $sql = "SELECT c.product, c.quantity, i.price 
                FROM {$this->table} c 
                JOIN inventory i ON c.product = i.product 
                WHERE c.session=?";
        $stmt = $GLOBALS['db']->prepare($sql);
        $stmt->execute([$session]);
        $total = 0;
        while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
            $total += $row['price'] * $row['quantity'];
        }
        return $total;
    }

    // 取得詳細明細（含商品資訊）
    public function display_contents($session) {
        $sql = "SELECT c.product, c.quantity, i.price, i.description
                FROM {$this->table} c
                JOIN inventory i ON c.product = i.product
                WHERE c.session=?
                ORDER BY c.id";
        $stmt = $GLOBALS['db']->prepare($sql);
        $stmt->execute([$session]);
        
        $contents = ['product'=>[], 'price'=>[], 'quantity'=>[], 'total'=>[], 'description'=>[]];
        $count = 0;
        while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
            $contents['product'][$count]     = $row['product'];
            $contents['price'][$count]       = $row['price'];
            $contents['quantity'][$count]    = $row['quantity'];
            $contents['total'][$count]       = $row['price'] * $row['quantity'];
            $contents['description'][$count] = $row['description'];
            $count++;
        }
        $contents['final'] = $this->cart_total($session);
        return $contents;
    }

    // 商品種類數
    public function num_items($session) {
        $sql = "SELECT COUNT(*) FROM {$this->table} WHERE session=?";
        $stmt = $GLOBALS['db']->prepare($sql);
        $stmt->execute([$session]);
        return $stmt->fetchColumn();
    }

    // 總數量
    public function quant_items($session) {
        $sql = "SELECT SUM(quantity) FROM {$this->table} WHERE session=?";
        $stmt = $GLOBALS['db']->prepare($sql);
        $stmt->execute([$session]);
        return (int)$stmt->fetchColumn();
    }
}
?>

前端調用範例

<?php
require 'Cart.php';
$cart = new Cart();
$session = $_COOKIE['scid'] ?? session_id();

// 加入商品
if (isset($_POST['add_product'])) {
    $cart->add_item($session, $_POST['product_id'], $_POST['qty'] ?? 1);
    header('Location: cart.php');
    exit;
}

// 顯示購物車
$items = $cart->display_contents($session);
?>
<table>
<tr><th>商品</th><th>單價</th><th>數量</th><th>小計</th><th>操作</th></tr>
<?php for ($i=0; $i