Using Google Open ID Connect to Access User's Gmail Account - google-apps-script

Question

How can I run a google apps script API on multiple gmail accounts? I currently have a script that accesses a user's gmail when it is authorized by the two files shown below. I store the refresh token for the client once the code asks for authorization. However, how can I use Open ID to get authorization for various users' accounts? Do I need to store a client id in addition to a refresh token to gain authorization for a user's gmail account? Thanks for taking the time to help in advance!
read_email.php
<?php
require_once '../google-api-php-client/src/Google/autoload.php';
function getEmails (){
session_start();
$client = new Google_Client();
$client->setAuthConfigFile('client_secrets.json');
$client->setScopes(array(
'https://mail.google.com/'
));
if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {
$client->setAccessToken($_SESSION['access_token']);
if($client->isAccessTokenExpired()){
header('Location: http://' . $_SERVER['HTTP_HOST'] . '/email_database/php/oauth2callback.php');
}
// Get the API client and construct the service object.
$service = new Google_Service_Script($client);
$scriptId = '**********************';
// Create an execution request object.
$request = new Google_Service_Script_ExecutionRequest();
set_time_limit(0);
$request->setFunction('test');
$response = $service->scripts->run($scriptId, $request);
$response = $response->getResponse();
$response = $response['result'];
return ($response);
} else {
$redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/email_database/php/oauth2callback.php';
header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}
}
?>
oauth2callback.php
<?php
require_once '../google-api-php-client/src/Google/autoload.php';
include 'functions.php';
$client = new Google_Client();
$client->setAuthConfigFile('client_secrets.json');
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/email_database/dashboard/php/oauth2callback.php');
$client->setScopes(array(
'https://mail.google.com/'
));
$client->setAccessType('offline');
$redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/email_database/dashboard/php/read_email.php';
$username = $_SESSION['username'];
$user = getUser($username);
if($user['refresh_token'] == null){
if (! isset($_GET['code'])) {
$auth_url = $client->createAuthUrl();
header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
} else {
//echo $_GET['code'];
$client->authenticate($_GET['code']);
$_SESSION['access_token'] = $client->getAccessToken();
$google_token = json_decode($_SESSION['access_token']);
print_r($google_token);
$refresh_token = $google_token->refresh_token;
addRefreshToken($username,$refresh_token);
//header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}
}
else {
$refresh_token = $user['refresh_token'];
$client->refreshToken($refresh_token);
$_SESSION['access_token']= $client->getAccessToken();
// header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}
?>