OrderValidationInterface.php

<?php

interface OrderValidationInterface
{
    public function validate(Order $order) : void;
}

class OrderHasAmount implements OrderValidationInterface
{
    public function validate(Order $order) : void
    {
        if (! $order->getAmount() > 0) {
            throw new Exception('Order amount must be more than zero.');
        }
    }
}

class OrderIsApproved implements OrderValidationInterface
{
    public function validate(Order $order) : void
    {
        if (! $order->isApproved()) {
            throw new Exception('Order is not approved.');
        }
    }
}

class OrderIsNotCancelled implements OrderValidationInterface
{
    public function validate(Order $order) : void
    {
        if (! $order->isCancelled()) {
            throw new Exception('Cancelled order cannot be processed.');
        }
    }
}