woocommerce liqpay Запрос не отрабатывает

После нажатия на кнопку, происходит редирект на страницу "Заказ принят", а не на liqpay.

PHP:

$private_key = "private_key ";
$public_key = "public_key ";
$json_string = json_encode(
    array(
        "version" => "3",
        "public_key" => $public_key,
        "amount" => "1",
        "currency" => "UAH",
        "description" => "Назначение платежа",
        "type" => "buy",
        "order_id" => date("d/m/Y-H:i:s"),
        "sandbox" => "1"
    )
);
$data = base64_encode($json_string);
$signature = base64_encode(sha1(
    $private_key .
        $data .
        $private_key,
    1
));

HTML:

<form id="liqpay" method="POST" action="https://www.liqpay.com/api/checkout" accept-charset="utf-8">
    <input type="hidden" name="data" value="<? echo $data; ?>" />
    <input type="hidden" name="signature" value="<? echo $signature; ?>" />
    <input type="image" src="//static.liqpay.ua/buttons/p1ru.radius.png" />
</form>