Paypal IPN not returning VERIFIED

I am trying to get IPN from PayPal to work, but it keeps returning INVALID instead of VERIFIED and I can't see what I've done wrong.

At the moment, I have 2 php files using the sample code from paypal's developer site. The code seem to run fine, I am using the sandbox so I can see the payment being transfered, so I don't understand why I am getting INVALID.

Can someone have a look at the code as I can't see what I've done wrong.

1. basic_payment.php (the file which sends the payment data to paypal)

<?php
  require_once ("paypalplatform.php");

  $actionType = "PAY";
  $cancelUrl = "http://[my server details go here]/cancel.php";
  $returnUrl = "http://[my server details go here]/success.php";  
  $currencyCode = "GBP";
  $receiverEmailArray = array( '[email protected]' );
  $receiverAmountArray = array( '2' );
  $receiverPrimaryArray = array();
  $senderEmail = "";      
  $feesPayer = "";
  $ipnNotificationUrl = "http://[my server details go here]/ipn.php";
  $memo = "";     
  $pin = "";      
  $preapprovalKey = "";       
  $reverseAllParallelPaymentsOnError = "";        
  $trackingId = generateTrackingID(); 
  $receiverInvoiceIdArray = array( $trackingId );
  $resArray = CallPay ($actionType, $cancelUrl, $returnUrl, $currencyCode, $receiverEmailArray,
                          $receiverAmountArray, $receiverPrimaryArray, $receiverInvoiceIdArray,
                          $feesPayer, $ipnNotificationUrl, $memo, $pin, $preapprovalKey,
                          $reverseAllParallelPaymentsOnError, $senderEmail, $trackingId
  );
  $ack = strtoupper($resArray["responseEnvelope.ack"]);
  if($ack=="SUCCESS")
  {
      if ("" == $preapprovalKey)
      {
          $cmd = "cmd=_ap-payment&paykey=" . urldecode($resArray["payKey"]);
          RedirectToPayPal ( $cmd );
      }
      else
      {
          $payKey = urldecode($resArray["payKey"]);
          $paymentExecStatus = urldecode($resArray["paymentExecStatus"]);
      }
  } 
?>

2. ipn.php (the file which listens for paypals response)

<?php

    $ipn_post_data = $_POST;

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

    $request = curl_init();
    curl_setopt_array($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,
    ));

    $response = curl_exec($request);
    $status   = curl_getinfo($request, CURLINFO_HTTP_CODE);

    curl_close($request);

    $to = "[email protected]";
    $from = "[email protected]";
    $subject = "response";
    $message = "<pre>".print_r($status,true)." - ".print_r($response,true)."</pre>\n";

    $header = 'To: Oshirowanen <[email protected]>' . "\r\n";
    $header .= 'From: Me <[email protected]>' . "\r\n";

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

?>

The error messages:

Apache error log - No errors

Which is why I added the email stuff in ipn.php to try to capture something, and it is returning:

<pre>200 - INVALID</pre>

Can anyone see what I am doing wrong?

Solution

Got it working using the basic sample code 4b,
Cleared $ipnNotificationUrl = ""; from the basic sample code as I had a value in there which I added myself,
Created a seller account instead of a business pro account in sandbox,
Set the seller account to enable the ipn url,
Used the following PHP 5.2 sample code for the ipn listener
Added the 2 lines into the listener, as described here, the 2 lines can be seen below:
Downloaded the cacert.pem certificate to my server from here and put it in the same directory as the ipn listener:

The 2 lines mentioned in point 6:

CURLOPT_SSL_VERIFYPEER => TRUE,
CURLOPT_CAINFO => 'cacert.pem',

I have no idea why the sandbox business pro account does not let me set an ipn url, but the seller account does.