Checkout With Gift Certificate

I’m trying to get gift certificates to be checked out by them self’s, and if there’s any product with them for it to come up with a message saying so. This is what I have so far but it’s not working. Can any one help me.

if ($mode == 'checkout') {
foreach ($_SESSION[‘cart’][‘products’] as $key => $value) {
    if ($cart['gift_certificates']['products']) {
        if (count($_SESSION['cart']['products']) > 1) {
        	$single_gift_voucher[] = $value['product'];
        }
    }	    
}

if (!empty($single_gift_voucher)) {
	foreach ($single_gift_voucher as $value) {
          fn_set_notification('W', __('notice'), '<i>'.$value.' </i>'.__('og_item_single_check_out_only').' '.'<i> '.$value.'</i>');
	}
	return array(CONTROLLER_STATUS_REDIRECT, "checkout.cart");
}

}