Aplicação do princípio de responsabilidade única do SOLID.

srp-solid-refactor.php

<?php

class Order
{
    public function calculateTotalSum(){/*...*/}
    public function getItems(){/*...*/}
    public function getItemCount(){/*...*/}
    public function addItem($item){/*...*/}
    public function deleteItem($item){/*...*/}
}

class OrderRepository
{
    public function load($orderID){/*...*/}
    public function save($order){/*...*/}
    public function update($order){/*...*/}
    public function delete($order){/*...*/}
}

class OrderViewer
{
    public function printOrder($order){/*...*/}
    public function showOrder($order){/*...*/}
}

//Reference: https://www.apphp.com/tutorials/index.php?page=solid-principles-in-php-examples