How can I form a free kassa request without the sign field in php?

Question

PHP

Hello, please help, tell me how you can create a free kassa forum so that customers enter the amount for any amount and not a specific one, here is the free kassa code

<?php
$merchant_id = '7012';
$secret_word = 'secret';
$order_id = '154';
$order_amount = '100.11';
$sign = md5($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='<?php=$merchant_id?>'>
<input type='hidden' name='oa' value='<?php=$order_amount?>'>
<input type='hidden' name='o' value='<?php=$order_id?>'>
<input type='hidden' name='s' value='<?php=$sign?>'>
<input type='hidden' name='i' value='1'>
<input type='hidden' name='lang' value='ru'>
<input type='hidden' name='us_login' value='<?php=$user['login']?>'>
<input type='submit' name='pay' value='Оплатить'>
 </form>

the sign field interferes, I can’t get a certain amount at the same time both in the form and in php

Answer 1

Obviously, to form $signon the server and then redirect the user to the payment gate with a valid value.
The main thing - do not try to form a request signature on the frontend - give the attacker a secret word.