<?
// hash отправляемых данных с ключем
$sign = md5($pay_cost .':'. $LMI_PAYEE_PURSE .':'. $pay_id .':'. $SECRET_KEY);
?>

<form action="https://merchant.webmoney.ru/lmi/payment.asp" method="POST">
  <fieldset>
    <input type="hidden" name="LMI_PAYMENT_AMOUNT" value="<?= $pay_cost?>">
    <input type="hidden" name="LMI_PAYMENT_DESC_BASE64" value="<?= base64_encode($pay_desc)?>">
    <input type="hidden" name="LMI_PAYEE_PURSE" value="<?= $LMI_PAYEE_PURSE?>">
    <input type="hidden" name="LMI_PAYMENT_NO" value="<?= $pay_id?>">
    <input type="hidden" name="hash" value="<?= $sign?>">
   <button type="submit" class="btn btn-success"><?= __('Оплатить')?> <?= $pay_cost?> WMR</button>
  </fieldset>
</form>