如何在数据库中添加IPN变量如POSTURL和USER ID Wordpress PHPmyadmin

Currently I have integrate the paypal with my site and this is working after successful payment. I have set IPN so that this can be update in my Database. But in database only few variable are inserted like PAYMENT_ID, Payment_Status,TXN_ID, Country_Code. This is code for inserting this data in database.

//Payment data
    $item_number = $_POST['item_number'];
    $txn_id = $_POST['txn_id'];
    $payment_gross = $_POST['mc_gross'];
    $currency_code = $_POST['mc_currency'];
    $payment_status = $_POST['payment_status'];

    //Check if payment data exists with the same TXN ID.
    $prevPayment = $db->query("SELECT payment_id FROM payments WHERE txn_id = '".$txn_id."'");
    if($prevPayment->num_rows > 0){
        exit();
    }else{
        //Insert tansaction data into the database
        $insert = $db->query("INSERT INTO payments(item_number,txn_id,payment_gross,currency_code,payment_status) VALUES('".$item_number."','".$txn_id."','".$payment_gross."','".$currency_code."','".$payment_status."')");
    }

I want to add User id and PostURL of the Post from where this payment is made.

Or some any unique identifier so that I can cross which user have make which post payment .