define('PAYPAL_TOKEN', '123456981723dasdas_paypal_token');
define('PAYPAL_URL', 'https://www.paypal.com/cgi-bin/webscr');

$tx = (isset($_GET['tx'])) ? $_GET['tx'] : '';

// Send POST for data response
$ch = curl_init(PAYPAL_URL);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,'cmd=_notify-synch&tx='.$tx.'&at='.PAYPAL_TOKEN);
curl_setopt($ch, CURLOPT_HEADER, 0);  // DO NOT RETURN HTTP HEADERS
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);  // RETURN THE CONTENTS OF THE CALL
$res = curl_exec($ch);

// Log response
$fp = fopen(PATH.'/log/paypal.log', 'a');
fwrite($fp, "\n".date('d.m.Y H:i')."\n");
fwrite($fp, $res);
fclose($fp);

// Parse response
$lines = explode("\n", $res);
$processed = (strcmp($lines[0], "SUCCESS") == 0) ? true : false;
unset($lines[0]);
unset($lines[30]);
$keyarray = array();
foreach ($lines as $line)
{
	list($key,$val) = explode("=", $line);
	$keyarray[urldecode($key)] = urldecode($val);
}

if ($processed && $keyarray['payment_status'] == 'Completed') {
    echo "OK";
} else {
    echo "Something went wrong.<br /> PayPal response: <br />".$res;
}

Process paypal response

PHP