form.html

<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
	<input name="amount" type="hidden" value="COMBIEN_CA_COUTE" />
	<input name="currency_code" type="hidden" value="EUR" />
	<input name="shipping" type="hidden" value="0.00" />
	<input name="tax" type="hidden" value="0.00" />
	<input name="return" type="hidden" value="URL_DE_SUCCES" />
	<input name="cancel_return" type="hidden" value="URL_ANNULATION" />
	<input name="notify_url" type="hidden" value="URL_NOTIFICATION_PAIEMENT" />
	<input name="cmd" type="hidden" value="_xclick" />
	<input name="business" type="hidden" value="IDENTIFIANT_VENDEUR" />
	<input name="item_name" type="hidden" value="CE QUE JE VENDS" />
	<input name="no_note" type="hidden" value="1" />
	<input name="lc" type="hidden" value="FR" />
	<input name="bn" type="hidden" value="PP-BuyNowBF" />
	<input name="custom" type="hidden" value="var1=1&var2=lol" />
	<input type="submit" value="S'abonner" class="btn primary">
</form>

ipn.php

<?php 
//permet de traiter le retour ipn de paypal
$email_account = "IDENTIFIANT@VENDEUR.COM";
$req = 'cmd=_notify-validate';

foreach ($_POST as $key => $value) {
    $value = urlencode(stripslashes($value));
    $req .= "&$key=$value";
}

$header = "POST /cgi-bin/webscr HTTP/1.0\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($req) . "\r\n\r\n";
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
$item_name = $_POST['item_name'];
$item_number = $_POST['item_number'];
$payment_status = $_POST['payment_status'];
$payment_amount = $_POST['mc_gross'];
$payment_currency = $_POST['mc_currency'];
$txn_id = $_POST['txn_id'];
$receiver_email = $_POST['receiver_email'];
$payer_email = $_POST['payer_email'];
parse_str($_POST['custom'],$custom);

if (!$fp) {

} else {
fputs ($fp, $header . $req);
while (!feof($fp)) {
    $res = fgets ($fp, 1024);
    if (strcmp ($res, "VERIFIED") == 0) {
        // vérifier que payment_status a la valeur Completed
        if ( $payment_status == "Completed") {
               if ( $email_account == $receiver_email) {
                /**
                 * C'EST LA QUE TOUT SE PASSE
                 * PS : tjrs penser à vérifier la somme !!
                 */
                

                /**
                 * FIN CODE
                 */
               }
        }
        else {
                // Statut de paiement: Echec
        }
        exit();
   }
    else if (strcmp ($res, "INVALID") == 0) {
		// Transaction invalide
    }
}
fclose ($fp);
}