PHP装饰者模式

interface Beverage
{
public function cost();
}

//被装饰
class Coffee implements Beverage
{
public function cost()
{
return 1;
}
}

//装饰者
class CondimentDecorator implements Beverage
{
public function cost()
{

}
}

class Milk extends CondimentDecorator
{
private $_beverage;
public function __construct($beverage)
{
if($beverage instanceof Beverage) {
$this->_beverage = $beverage;
}
}

public function cost()
{
return $this->_beverage->cost() + 2;
}
}

class Sugar extends CondimentDecorator
{
private $_beverage;

public function __construct($beverage)
{
if($beverage instanceof Beverage)
{
$this->_beverage = $beverage;
}
}

public function cost()
{
return $this->_beverage->cost()+3;
}
}

$coffee = new Coffee();
//加牛奶
$coffee = new Milk($coffee);
//加糖
$coffee = new Sugar($coffee);

//总消费
echo $coffee->cost();