<?php
/* --------------------------------------------------------------
 ec_proxy.php 2019-10-23
 Gambio GmbH
 http://www.gambio.de
 Copyright (c) 2019 Gambio GmbH
 Released under the GNU General Public License (Version 2)
 [http://www.gnu.org/licenses/gpl-2.0.html]
 --------------------------------------------------------------
 */

declare(strict_types=1);

function getClientIp() {
    $remoteAddressKey = 'REMOTE_ADDR';
    $proxyHeader      = 'HTTP_X_FORWARDED_FOR';
    
    if (!array_key_exists($remoteAddressKey, $_SERVER)) {
        return null;
    }
    
    if (array_key_exists($proxyHeader, $_SERVER)) {
        $proxies = explode(',', $_SERVER[$proxyHeader]);
        
        $clientIp = $proxies[0];
        
        if (filter_var($clientIp, FILTER_VALIDATE_IP)) {
            return $clientIp;
        }
    }
    
    return $_SERVER[$remoteAddressKey];
}

$query = $_GET;

// parses the google path to extract the version query param
$gPath       = $query['prx'];
$parsedGPath = parse_url($gPath);
if (array_key_exists('query', $parsedGPath)) {
    $querySegments = explode('=', $parsedGPath['query']);
    $query         = array_merge([$querySegments[0] => $querySegments[1]], $query);
}
unset($query['prx']);

// creates the final google analytics url
$gUrl = 'https://www.google-analytics.com' . $parsedGPath['path'];

// try to fetch the client ip
$clientIp = getClientIp();
if ($clientIp) {
    $query['uip'] = $clientIp;
}
if (array_key_exists('HTTP_USER_AGENT', $_SERVER)) {
    $query['ua'] = $_SERVER['HTTP_USER_AGENT'];
}

// sends the analytics data to the google servers
$finalUrl  = $gUrl . '?' . http_build_query($query);
$gCurl     = curl_init($finalUrl);
curl_setopt($gCurl, CURLOPT_RETURNTRANSFER, true);
$gResponse = curl_exec($gCurl);
curl_close($gCurl);

header('Content-Type: image/gif');
echo $gResponse;