To receive notifications about purchases on the forum, you need to go to https://opencartforum.com/clients/info/ 
add Callback URL and Secret Key. On the side of your server, accept data, you can test the work in the personal data section

Example:

<?php 
namespace OpenCartForum\API;

class Purchases 
{
	private $private_key;

	private static $instance = null;
	private $headers = [];

	public static function getInstance($private_key)
	{
		if (self::$instance !== null) 
		{
			return self::$instance;
		}

		self::$instance = new self($private_key);

		return self::$instance;
	}

	public function getPurchase() 
	{
		return $this->auth() ? $this->getForm() : false;
	}

	private function __construct($private_key) 
	{
		$this->private_key = $private_key;
	}

	private function getForm() 
	{
		if (empty($this->form)) {
			$this->form = json_decode(file_get_contents('php://input'), true);
		}

		return $this->form;
	}

	private function auth() 
	{
		if (empty($form = $this->getForm()) || empty($form['hash']) || empty($form['order']) ) {
			$this->addHeader("{$_SERVER['SERVER_PROTOCOL']} 400 Bad Request");
			return false;

		} 

		if ( ! hash_equals( hash_hmac('md5', (\strlen($form['order']['id']) . $form['order']['id'] 
			. \strlen($form['order']['date']) . $form['order']['date']), $this->private_key), $form['hash']) ) {
			
			$this->addHeader("{$_SERVER['SERVER_PROTOCOL']} 401 Unauthorized");
			$this->addHeader('Content-Type: application/json; charset=UTF-8');
			$this->addHeader('State: Unauthorized');
			return false;
		}

		$this->addHeader("{$_SERVER['SERVER_PROTOCOL']} 200 OK");
		$this->addHeader('Content-Type: application/json; charset=UTF-8');

		switch ($form['status']) {
			case 'auth':
				$this->addHeader('State: Authorized');
				return false;
			
			case 'success':
				$this->addHeader('State: Received');
				return true;
		}
	}

	private function addHeader($header) 
	{
		$this->headers[] = $header;
	}

	private function output(array $data = []) 
	{
		if (!headers_sent()) {
			foreach ($this->headers as $header) {
				header($header, true);
			}
		}

		echo json_decode($data);
		die;
	}

	public function __destruct() 
	{
		$this->output();
	}
}

/* Cекретный ключ, указаный на форуме в разделе https://opencartforum.com/clients/info/, там же укажите Callback URL */
$private_key		= "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
$opencartforum_api 	= \OpenCartForum\API\Purchases::getInstance($private_key);

if (($purchase_info = $opencartforum_api->getPurchase())) {
	$file = $_SERVER['DOCUMENT_ROOT'] . '/ocf-purchase.log';
	
	$f = fopen($file, 'w+');
	fwrite($f, print_r($purchase_info, true));
	fclose($f);
}

The response must contain mandatory headings::

Content-Type: application/json; charset=UTF-8

State: Unauthorized - Неуспешная авторизация
State: Authorized - Успешная авторизация, готов получить форму
State: Received	- Форма успешно принята

We send two requests, the first authorization request, the second request with a form. The HTTP status code should be 200.

Sales messages will have the following structure:

Array
(
    [marketplace] => (string) opencartforum
    [hash] => (string) 74e6f7298a9c2XXXXXXXXXXXXX,
    [status] => (string) success | auth,
    [order] => Array
        (
            [id] => (int) 1090XXX
            [date] => (int) 1614040731
            [domain] => (strinng) domain.com
            [test_domain] => (strinng) test.domain.com
            [options]=> array (
                    [1]=> array (
                      ["cost"]=> string(7) 2000.00
                      ["name"]=> string(36) Установка: 2 000.00 руб
                    )
            	)
            [discount]=> array (
                  [amount]=> string(4) 1260 // Price 
                  [name]=>  string(112) (string) File - 10%
                )
            [coupon]=>  array (
                        [code]=> string(14) 1237-promo2021
                        [discount]=> string(4) 1344 // Price 
                        [text]=>  string(16) 1344.00 руб
                      )
            [total] => Array
                (
                    [currency] => (string) RUB  // Или USD
                    [amount] => (float) 300
                )
            [quantity]   => (int) 1
            [commission] => (int) 30
        )

    [developer] => Array
        (
            [id] => (int) XXXX
            [name] => (string) developer
            [email] => (string) [email protected]
            [link] => (string) https://opencartforum.com/profile/XXXX-developer/
            [credits] => Array
                (
                    [0] => Array
                        (
                            [currency] => (string) RUB
                            [amount] => (float) 6939.46
                        )

                    [1] => Array
                        (
                            [currency] => (string) USD
                            [amount] => (float) 3701.44
                        )

                )

        )

    [customer] => Array
        (
            [id] => (int) XXXX
            [name] => (string) Customer
            [email] => (string) [email protected]
            [link] => (string) https://opencartforum.com/profile/XXXX-customer/
        )

    [file] => Array
        (
            [id] => (int) XXXX
            [name] => (string) File
            [basePrice] = Array 
                         ( 
                                [0] => Array 
                              ( 
                                [currency] => (string) RUB 
                                [amount] => (float) 750.00 
                              ) 
                                [1] => Array 
                              ( 
                                [currency] => (string) USD 
                                [amount] => (float) 10.00
                              ) 
                          )
            [link] => (string) https://opencartforum.com/files/file/XXXX-file/
        )

)

Discussion of the forum API takes place in the topic: