Как упоминалось в одном из комментариев в ответе ниже, я попыталсяэтот урок, Итак, теперь у меня есть следующее:

The ipn.php file:

<?php

    $ipn_post_data = $_POST;

    $url = 'https://www.sandbox.paypal.com/cgi-bin/webscr';

    // Set up request to PayPal
    $request = curl_init();
    curl_setopt_array($request, array
    (
        CURLOPT_URL => $url,
        CURLOPT_POST => TRUE,
        CURLOPT_POSTFIELDS => http_build_query(array('cmd' => '_notify-validate') + $ipn_post_data),
        CURLOPT_RETURNTRANSFER => TRUE,
        CURLOPT_HEADER => FALSE,
        CURLOPT_SSL_VERIFYPEER => TRUE,
        CURLOPT_CAINFO => 'cacert.pem',
    ));

    // Execute request and get response and status code
    $response = curl_exec($request);
    $status   = curl_getinfo($request, CURLINFO_HTTP_CODE);

    // Close connection
    curl_close($request);

    if($status == 200 && $response == 'VERIFIED')
    {
        $subject = "valid";
        $message = "good";
    }
    else
    {
        $subject = "invalid";
        $message = "bad";
    }

    $to = "[email protected]";
    $from = "[email protected]";

    $header  = 'MIME-Version: 1.0' . "\r\n";
    $header .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
    $header .= 'To: Oshirowanen <[email protected]>' . "\r\n";
    $header .= 'From: Me <[email protected]>' . "\r\n";

    mail($to,$subject,$message,$header);

?>

The received email:

Subject "invalid"
Message "bad"