/**
     * This routine will pay for any advert placements with available credit.
     * You are redirected here after payments with paypal or if you already have enough credit
     */
    private function pay_for_adverts_with_credit()
    {
        $errors = array();
        // Ok lets get all the unpaid ad placements for the given user
        $ad_placements = $this->advert_model->getUnpaidAdPlacementsForUser($user->user_id);
        $user_id = $this->user_model->getLoggedInUser()->user_id;
        foreach($ad_placements as $ad_placement)
        {
            // Let's try to purchase them with credit
            if (!$this->user_model->deductCredit($user_id, $ad_placement->cost))
            {
                $errors[] = 'Failed to pay for ad placement scheduled at: ' . $ad_placement->scheduled . ' as you do not have enough credit';
                break;
            }

            $this->advert_model->markAdvertPlacementActive($ad_placement->advert_placement_id);
        }

        if (count($errors) > 0)
        {
            print_r($errors);
        }
    }