How to find user's email ID? - Google Oauth2 API PHP Client - google-oauth

Question

I am implementing an Audio library manager which uses user's google drive as backend storage. I am using PHP (google-api-php-client-2.2.1) to get the user's authorization for accessing his/her google drive.
So far I am able to receive access token and refresh token using example code from google (https://developers.google.com/api-client-library/php/auth/web-app), and also tested by listing files of user's google drive.
However, I am not able to figure out programmatically which user has given authorization to my application. Just user's email address will do. I don't need any other information.
I am using the following code.
<?php
require_once 'google-api-php-client/vendor/autoload.php';
session_start();
$client = new Google_Client();
$client->setAuthConfig('client_secret.json'); //downloaded from google oauth credentials portal. It includes redirect URL
$client->setAccessType("offline"); // offline access
$client->setIncludeGrantedScopes(true); // incremental auth
$client->addScope(Google_Service_Drive::DRIVE_METADATA_READONLY);
if (! isset($_GET['code'])) {
//Get authorization and User consent from Google
$auth_url = $client->createAuthUrl();
header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
} else { //Authorization completed
//Get access token
$client->authenticate($_GET['code']);
$_SESSION['access_token'] = $client->getAccessToken();
$_SESSION['refresh_token'] = $client->getRefreshToken();
echo "Access Token<br>";
var_dump ($_SESSION['access_token']); //works
echo "<br>";
echo "Refresh Token<br>";
var_dump ($_SESSION['refresh_token']); //works
echo "<br>";
$service=new Google_Service_Oauth2($client);
echo "Auth2 Service created<br>"; //Getting this print
$usr_info=$service->userinfo;
echo "User Info<br>"; //Getting this print
var_dump ($usr_info); //Getting this print, Very long JSON structure. But info is mostly pertaining to my app...no user information showing here
echo "<br>";
//$usr_info_get = $usr_info->get(); //Throwing Error
//$usr_info_get = $usr_info->get(['email']); //Throwing Error
//$usr_info_get = $usr_info->get()['email']; //Throwing Error
echo "User Info Get<br>";
//var_dump ($usr_info_get);
echo "<br>";
}
?>
The class for userInfo is like this
class Google_Service_Oauth2_Resource_Userinfo extends Google_Service_Resource
{
/**
* (userinfo.get)
*
* #param array $optParams Optional parameters.
* #return Google_Service_Oauth2_Userinfoplus
*/
public function get($optParams = array())
{
$params = array();
$params = array_merge($params, $optParams);
return $this->call('get', array($params), "Google_Service_Oauth2_Userinfoplus");
}
}
The params for get() method should be optional, but
$userInfo->get()
also given me Server Error.
I am not finding anything else from my google search on this.
I tried this (6 year old thread) but code errors out (apiHttpRequest class not found)
How to identify a Google OAuth2 user?
I tried reading about openID but seems kinda complex. Also, not sure if google-api-php-client has helper class for openID.
I would really appreciate your help. Already spent 2+ days reading google api client php code but now up against the wall.