无法调试输出

今天我正在阅读设计模式，我试图制作一个示例程序，它包含一个接口，两个实现该接口的类和一个主索引类。首先是界面

<?php 
interface Iproduct 
{
    //Define the abstract method
    public function apple();
    public function mango();    
}

实现接口的两个类

<?php 
// Including the interface
include_once 'Iproduct.php';
    class Apple implements Iproduct 
    {
        public function apple()
        {
            echo ("We sell apples!");
        }   
        public function mango()
        {
            echo ("We do not sell Mango!");
        }
    }
<?php
// Include the interface Iprodduct
include_once 'Iproduct.php';
class Mango implements Iproduct
{
    public function apple()
    {
        echo ("We do not sell Apple");
    } 
    public function mango()
    {
        echo ("We sell mango!");    
    }
}

现在是主类

<?php
include_once ('apple.php');
include_once ('Mango.php');
class UserProduct
{
    public function __construct()
    {
        $apple_class_obj=new Apple();
        $mango_class_obj=new Mango();
        //echo("<br/> the apple class object: ".$apple_class_obj);
    }   
}
//creating the object of the UserProduct
echo ("creating the object!<br/>");
$userproduct_obj=new UserProduct();
?>

执行代码时得到的输出是：

creating the object!
we sell apples!we sell mango

现在的问题是我无法理解第二个输出如何，即，我们卖苹果！我们卖芒果！正在显示。请让我知道原因