<?php
require_once __DIR__.'/vendor/autoload.php';

session_start();

$client = new Google_Client();
$client->setAuthConfigFile('client_secret.json');
$client->setRedirectUri('http://localhost/search/index.php/');
$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);
$client->setAccessType("offline");

if (! isset($_GET['code'])) {
  $auth_url = $client->createAuthUrl();
  header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
} else {
  $client->authenticate($_GET['code']);
  $_SESSION['access_token'] = $client->getAccessToken();
  $redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/';
  header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}

<?php
require_once __DIR__.'/vendor/autoload.php';

session_start();

$client = new Google_Client();
$client->setAuthConfig('client_secret.json');
$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);

if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {
    print_r($_SESSION);
  $client->setAccessToken($_SESSION['access_token']);
  $drive = new Google_Service_Drive($client);
  $files = $drive->files->listFiles(array())->getItems();
  echo json_encode($files);
} else {
  $redirect_uri = 'http://localhost/search/login.php/';
    // echo "I am in else.";
  header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}