Paypal IPN not working, cannot verify - php

Question

I am testing Paypal IPN using sandbox and it is not working. The result always shows INVALID and i have no idea why. Please kindly help me to check if there were any error in the coding.
(Everytime i send an IPN, there are no data recorded in the IPN history.)
<?php
class Paypal_IPN
{
/** #var string $_url The paypal url to go to through cURL;
/**
* #param string $mode 'live' or 'sandbox'
*/
public function __construct($mode = 'live')
{
if ($mode == 'live')
$this->_url = 'https://www.paypal.com/cgi-bin/webscr';
else
$this->_url = 'https://www.sandbox.paypal.com/cgi-bin/webscr';
}
public function run()
{
$postFields = 'cmd=_notify-validate';
foreach($_POST as $key => $value)
{
$postFields .= "&$key=".urlencode($value);
}
$ch = curl_init();
curl_setopt_array($ch, array(
CURLOPT_URL => $this->_url,
CURLOPT_RETURNTRANSFER => true,
CURLOPT_SSL_VERIFYPEER => false,
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => $postFields
));
$result = curl_exec($ch);echo curl_error($ch);
curl_close($ch);
$fh = fopen('result.txt', 'w');
fwrite($fh, $result . ' -- ' . $postFields);
fclose($fh);
echo $result;
}
}
?>
<?php
require 'Paypal_IPN.php';
$paypal = new Paypal_IPN('sandbox');
$paypal->run();
?>

Answer 1

Here's my loop for repopulating the string. I've tweaked it over the years to deal with oddities. Also, remember the final string must be encoded UTF8
foreach ($_POST as $key => $value) {
$value = urlencode(stripslashes($value));
//Fixes some special characters Paypal sends
$value = preg_replace('/(.*[^%^0^D])(%0A)(.*)/i','${1}%0D%0A${3}', $value);
$req .= '&' . $key . '=' . $value;
}