OrderNotificationInterface.php

<?php

interface OrderNotificationInterface
{
    public function notify(Order $order) : void;
}

class SlackNotification implements OrderNotificationInterface
{
    public function __construct(Slack $slack) {
        $this->slack = $slack;
    }

    public function notify(Order $order) : void
    {
        $message = $this->getMessage($order);
        $this->slack->to('#order')->send($message);
    }

    protected function getMessage(Order $order) : string
    {
        $format   = '@%s Order has been processed. OrderID: %d';
        $userName = $order->getUserName();
        $message  = sprintf($format, $userName, $order->getId());
        
        return $message;
    }
}