如何从paypal IPN处理POST变量

Question

目前，我正在使用此处的代码访问我的sanbox环境：https://github.com/paypal/ipn-code-samples/blob/master/paypal_ipn.php

Evertyhing运行顺畅，清晰，我在IPN历史网站上检索邮件，我已将所有内容保存在我的日志文件中。

问题在于：

if (strcmp ($res, "VERIFIED") == 0) {
    // check whether the payment_status is Completed
    // check that txn_id has not been previously processed
    // check that receiver_email is your PayPal email
    // check that payment_amount/payment_currency are correct
    // process payment and mark item as paid.
    // assign posted variables to local variables
    //$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'];

    if(DEBUG == true) {
        error_log(date('[Y-m-d H:i e] '). "Verified IPN: $req ". PHP_EOL, 3, LOG_FILE);
    }
}

如果我想要回显$payer_email变量，则没有任何反应。如果我想将这些变量存储在我的数据库中 - 没有任何反应。

似乎这些变量是空的，如果它们确实存在，我如何获取这些变量并根据我的需要处理它们。