Google OAuth2 refresh token using PHP - php

Question

I am trying to create and save an access token that I can reference when other users in my organization (who do not have API access) log in. For instance I need to call Google Admin API to grab their orgUnit. When I get my access_token I can use this with other users but it expires after an hour and I have to re-log in with my account to get a new token.
In Short, I am not sure where or how to create the refresh token so I can save it in my datbase.
//Create Client Request to access Google API
$client = new Google_Client();
$client->setApplicationName(APPLICATION_NAME);
$client->setClientId(CLIENT_ID);
$client->setClientSecret(CLIENT_SECRET);
$client->setRedirectUri(REDIRECT_URL);
$client->setDeveloperKey(API_KEY);
$client->setAccessType('offline');
$client->addScope("https://www.googleapis.com/auth/userinfo.email");
$client->addScope("https://www.googleapis.com/auth/admin.directory.user.readonly");
//Send Client Request
if($client->isAccessTokenExpired()){
//not doing anything here at the moment
}
$objOAuthService = new Google_Service_Oauth2($client);
//Logout
if (isset($_REQUEST['logout'])) {
unset($_SESSION['access_token']);
unset($_SESSION['code']);
$client->revokeToken();
header('Location: ' . filter_var(REDIRECT_URL, FILTER_SANITIZE_URL)); //redirect user back to page
}else if (isset($_GET['code'])) {
//Authenticate code from Google OAuth Flow
//Add Access Token to Session
$client->authenticate($_GET['code']);
$_SESSION['code'] = $_GET['code'];
$_SESSION['access_token'] = $client->getAccessToken();
header('Location: ' . filter_var(REDIRECT_URL, FILTER_SANITIZE_URL));
}
//Set Access Token to make Request
if (isset($_SESSION['access_token'])) {
$client->setAccessToken($_SESSION['access_token']);
}