<?php
$iduseraa   = "" . $_SESSION['user']['identity'] . "";
$id_usera   = explode("id", $iduseraa);
$loginik      = $id_usera[1];
$merchant_id = '37941';
$secret_word = 'q6xhia4i';
$user_id = $loginik;
$order_id = rand(1000,99999);
$order_amount = $_POST['oa'];
$sign = md5($merchant_id.':'.$order_amount.':'.$secret_word.':'.$order_id);

?>
<form method='get' action='http://www.free-kassa.ru/merchant/cash.php'>
    <input type='hidden' name='m' value='<?=$merchant_id?>'>
    <input type='number' max="1000" min="5" name='oa' value="10" style="width: 300px;margin-top: 7px;">
    <input type='hidden' name='us_vkid' value='<?=$user_id?>'>
    <input type='hidden' name='o' value='<?=$order_id?>'>
    <input type='hidden' name='s' value='<?=$sign?>'><br><br>
    <input type='submit' name='pay' value='Оплатить'>
</form>