如果你的系统需要得知PayPal支付的最终结果（成功、失败），那么PayPal的IPN接口是最简单的解决方案。

<form action="https://www.sandbox.paypal.com/cgi-bin/websc" method="post">
  <input type="hidden" name="business" value="seller_1316102076_biz@qq.com">
  <input type="hidden" name="item_name" value="海底捞优惠券">
  <input type="hidden" name="amount" value="30">
  <input type="hidden" name="no_note" value="1">
  <input type="hidden" name="return"  value="http://adsl.redicecn.com/order.php?status=ok">
  <input type="hidden" name="cancel_return" value="http://adsl.redicecn.com/">
  <input type="hidden" name="custom" value="1">
  <input type="hidden" name="notify_url" value="http://adsl.redicecn.com/ipn.php">
  
  <input type="hidden" name="cmd" value="_xclick">
  <input type="hidden" name="currency_code" value="CAD">
  <input type="hidden" name="charset" value="utf-8" />
  <input type="hidden" name="rm" value="1" />
 <input type="submit" value="支付" />
  </form>

<?

// read the post from PayPal system and add 'cmd'
$req = 'cmd=_notify-validate';

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

// post back to PayPal system to validate
$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, $errstr, 30); // 沙盒用
//$fp = fsockopen ('ssl://www.paypal.com', 443, $errno, $errstr, 30); // 正式用

// assign posted variables to local variables
$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'];
$mc_gross = $_POST['mc_gross ']; // 付款金额
$custom = $_POST['custom ']; // 得到订单号

if (!$fp) {
// HTTP ERROR
} else {
fputs ($fp, $header . $req);
while (!feof($fp)) {
$res = fgets ($fp, 1024);
if (strcmp ($res, "VERIFIED") == 0) {
// check the payment_status is Completed
// check that txn_id has not been previously processed
// check that receiver_email is your Primary PayPal email
// check that payment_amount/payment_currency are correct
// process payment
// 验证通过。付款成功了，在这里进行逻辑处理（修改订单状态，邮件提醒，自动发货等）
}
else if (strcmp ($res, "INVALID") == 0) {
// log for manual investigation
// 验证失败，可以不处理。
}
}
fclose ($fp);
}
?>