How to accept and process XML from PrivatBank in Drupal 7?

Question

PHP

Hello!
Can you tell me how difficult to create a Drupal 7 module I am describing?
There is an "Invoice" node, it has the following fields:
- field_payment_id
- field_payment_price
- field_payment_status
Based on these data, payment is generated in Privat24 by the form

<form method="POST" action="https://api.privatbank.ua/p24api/ishop">
      <input type="hidden" name="amt" value="<?php echo $field_payment_price ?>" />
      <input type="hidden" name="ccy" value="UAH" />
      <input type="hidden" name="merchant" value="12345" />
      <input type="hidden" name="order" value="<?php echo $field_payment_id ?>" />
      <input type="hidden" name="details" value="Детали платежя" />
      <input type="hidden" name="ext_details" value="Описание" />
      <input type="hidden" name="pay_way" value="privat24" />
      <input type="hidden" name="return_url" value="http://..." />
      <input type="hidden" name="server_url" value="http://..." />
      <button formtarget="_blank" class="privat" type="submit"></button>
  </form>

After the client makes a payment, the module, as I understand it, should send the following XML request

<?xml version="1.0" encoding="UTF-8"?>
    <request version="1.0">
        <merchant>
            <id>12345</id>
            <signature>d66d192cb123456788e08cfe1ca171535</signature>
        </merchant>
        <data>
            <oper>cmt</oper>
            <wait>0</wait>
            <test>0</test>
            <payment>
                <prop name="id" value="1234567" />
                <prop name="ref" value="P24A02509023364489" />
            </payment>
        </data>
    </request>

The request signature is calculated as follows (PHP): $sign=sha1(md5($data.$password)); $data - content of the given request tag; $password - merchant's personal password received during registration.
And from Privat24 there should be an answer:

<?xml version="1.0" encoding="UTF-8"?>
  <response version="1.0">
    <merchant>
      <id>12345</id>
      <signature>37aad972d7461234567893802230be38</signature>
    </merchant>
    <data>
      <oper>cmt</oper>
      <payment id="1234567" status="ok" message="Исполнен" ref="P24A02509023364480" />
    </data>
  </response>

Possible status values:
not found - payment not found;
ok - successful;
err - rejected;
snd - in processing
After the payment has been received (status="ok")
It is necessary to change the corresponding status in the node's $payment_status field in the node, in which the value of the $field_payment_id field corresponds to the incoming value
. Merchant id and other data are entered in the same place.
How complex can such a module be? Can you share some good advice on how to do this the easiest way? where to read the relevant documentation?

Answer 1

Thank you. Yes, I have already shoveled and picked it more than once. But so far no results. I'll try again. Thank you.
Question Resolved!

Answer 2

I think you need to find any payment and take it as a basis.
here it seems to be ready for Drupal
https://www.drupal.org/node/729116