OrderProcessor.php

<?php

class OrderProcessor
{
    public function __construct(Slack $slack) {
        $this->slack = $slack;
    }
    
    public function process(Order $order)
    {
        if (! $order->getAmount() > 0) {
            throw new Exception('Order amount must be more than zero.');
        }

        if (! $order->isApproved()) {
            throw new Exception('Order is not approved.');
        }

        if (! $order->isCancelled()) {
            throw new Exception('Cancelled order cannot be processed.');
        }

        $order->update();
        
        $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;
    }
}