<form action="https://money.yandex.ru/quickpay/confirm.xml" target="_blank" method="POST">
<input type="hidden" value="410011996888583" name="receiver">
<input type="hidden" value="" name="label">
<input type="hidden" value="Оплата вебинара" name="FormComment">
<input type="hidden" value="Оплата вебинара" name="short-dest">
<input type="hidden" value="false" name="writable-targets">
<input type="hidden" value="true" name="writable-sum">
<input type="hidden" value="false" name="comment-needed">
<input type="hidden" value="shop" name="quickpay-form">
<input type="hidden" value="1" name="mail">
</form>
<input type="hidden" value="1" name="mail">
$mail = $_POST['mail']; // user's mail