PHP开发商城，实现购物车优惠计算功能

class Cart {
   private $items; // 存储购物车中的商品
   public function __construct() {
       $this->items = array();
   }
   public function addItem($item) {
       // 将商品添加到购物车中
   }
   public function removeItem($item) {
       // 从购物车中移除商品
   }
   public function getItems() {
       // 获取购物车中的所有商品
   }
}

class DiscountRule {
   private $type; // 优惠类型
   private $condition; // 优惠条件
   private $discount; // 优惠金额/折扣比例
   private $giftItem; // 赠送商品
   public function __construct($type, $condition, $discount, $giftItem) {
       $this->type = $type;
       $this->condition = $condition;
       $this->discount = $discount;
       $this->giftItem = $giftItem;
   }
   public function getType() {
       return $this->type;
   }
   public function getCondition() {
       return $this->condition;
   }
   public function getDiscount($total) {
       // 根据优惠条件和优惠金额/折扣比例来计算优惠金额
   }
   public function getGiftItem() {
       return $this->giftItem;
   }
}

public function calculateDiscount($discountRules) {
   $total = $this->calculateTotal();
   $discount = 0;
   foreach ($discountRules as $discountRule) {
       $condition = $discountRule->getCondition();
       if ($total >= $condition) {
           $discount += $discountRule->getDiscount($total);
       }
   }
   return $discount;
}

public function applyDiscount($discountRules) {
   $discount = $this->calculateDiscount($discountRules);
   // 扣除优惠金额
   // 赠送赠品
}