Google API client authorization: Internal Server Error 500

I'm using an OAuth 2.0 for Web Server Applications example code on remote web server. $client->authenticate(...) causes Internal Server Error 500.

No idea why, can you help me please?

Here is the web address: http://ift.tt/2jlaWXz And here is the code:

//index.php

require_once(DIR ."/api/vendor/autoload.php");

session_start();

$client = new Google_Client();

$client->setAuthConfig('client_secrets.json');

$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);

if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {

$client->setAccessToken($_SESSION['access_token']);

$drive_service = new Google_Service_Drive($client);

$files_list = $drive_service->files->listFiles(array())->getItems();

echo json_encode($files_list);

} else {

$redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/gdrive/oauth2callback.php';

header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));

}

//oauth2callback.php

require_once DIR .'/api/vendor/autoload.php';

session_start();

$client = new Google_Client();

$client->setAuthConfigFile('client_secrets.json');

$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/gdrive/oauth2callback.php');

$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);

if (!isset($_GET['code'])) {

$auth_url = $client->createAuthUrl();

header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));

} else {

$client->authenticate($_GET['code']);

}