This sample application describes how your PHP application can connect to the user’s Gmail account using the Google PHP client library and OAuth2. You’ll need to create the application inside Google Console.

The Client ID and secret are stored in a separate JSON while the access token and refresh token are also stored in the local file system.

<?php
  require_once __DIR__ . '/vendor/autoload.php';
  session_start();
  date_default_timezone_set('America/Los_Angeles');

  $REDIRECT_URI = 'http://localhost:8080';
  $KEY_LOCATION = __DIR__ . '/client_secret.json';
  $TOKEN_FILE   = "token.txt";

  $SCOPES = array(
      Google_Service_Gmail::MAIL_GOOGLE_COM,
      'email',
      'profile'
  );

  $client = new Google_Client();
  $client->setApplicationName("ctrlq.org Application");
  $client->setAuthConfig($KEY_LOCATION);

  // Incremental authorization
  $client->setIncludeGrantedScopes(true);

  // Allow access to Google API when the user is not present.
  $client->setAccessType('offline');
  $client->setRedirectUri($REDIRECT_URI);
  $client->setScopes($SCOPES);

  if (isset($_GET['code']) && !empty($_GET['code'])) {
      try {
          // Exchange the one-time authorization code for an access token
          $accessToken = $client->fetchAccessTokenWithAuthCode($_GET['code']);

          // Save the access token and refresh token in local filesystem
          file_put_contents($TOKEN_FILE, json_encode($accessToken));

          $_SESSION['accessToken'] = $accessToken;
          header('Location: ' . filter_var($REDIRECT_URI, FILTER_SANITIZE_URL));
          exit();
      }
      catch (\Google_Service_Exception $e) {
          print_r($e);
      }
  }

  if (!isset($_SESSION['accessToken'])) {

      $token = @file_get_contents($TOKEN_FILE);

      if ($token == null) {

          // Generate a URL to request access from Google's OAuth 2.0 server:
          $authUrl = $client->createAuthUrl();

          // Redirect the user to Google's OAuth server
          header('Location: ' . filter_var($authUrl, FILTER_SANITIZE_URL));
          exit();

      } else {

          $_SESSION['accessToken'] = json_decode($token, true);

      }
  }

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

  /* Refresh token when expired */
  if ($client->isAccessTokenExpired()) {
      // the new access token comes with a refresh token as well
      $client->fetchAccessTokenWithRefreshToken($client->getRefreshToken());
      file_put_contents($TOKEN_FILE, json_encode($client->getAccessToken()));
  }

  $gmail = new Google_Service_Gmail($client);

  $opt_param               = array();
  $opt_param['maxResults'] = 10;

  $threads = $gmail->users_threads->listUsersThreads("amit@labnol.org", $opt_param);

  foreach ($threads as $thread) {
      print $thread->getId() . " - " . $thread->getSnippet() . '<br/>';
  }

?>