<?
// hash отправляемых данных с паролем
$sign = md5($MrchLogin .':'. $pay_cost .':'. $pay_id .':'. $sMerchantPass1 . ':shp_service=' . $pay_service . ':shp_service_id=' . $pay_service_id);
?>

<form action="https://auth.robokassa.ru/Merchant/Index.aspx" method="POST">
  <input type="hidden" name="MrchLogin" value="<?= $MrchLogin?>">
  <input type="hidden" name="OutSum" value="<?= $pay_cost?>">
  <input type="hidden" name="InvId" value="<?= $pay_id?>">
  <input type="hidden" name="Desc" value="<?= $pay_desc?>">
  <input type="hidden" name="SignatureValue" value="<?= $sign?>">
  <input type="hidden" name="shp_service" value="<?= $pay_service?>">
  <input type="hidden" name="shp_service_id" value="<?= $pay_service_id?>">
   <button type="submit" class="btn btn-success"><?= __('Оплатить')?> <?= $pay_cost?> руб.</button>
</form>