Google Gmail API - How to login programatically?

// Load the Google API PHP Client Library.
require_once __DIR__ . '/vendor/autoload.php';
// Use the developers console and download your service account
// credentials in JSON format. Place the file in this directory or
// change the key file location if necessary.
putenv('GOOGLE_APPLICATION_CREDENTIALS='.__DIR__.'/service-account.json');
/**
 * Gets the Google client refreshing auth if needed.
 * Documentation: https://developers.google.com/identity/protocols/OAuth2ServiceAccount
 * Initializes a client object.
 * @return A google client object.
 */
function getGoogleClient() {
    return getServiceAccountClient();
}
/**
 * Builds the Google client object.
 * Documentation: https://developers.google.com/api-client-library/php/auth/service-accounts
 * Scopes will need to be changed depending upon the API's being accessed. 
 * array(Google_Service_Analytics::ANALYTICS_READONLY, Google_Service_Analytics::ANALYTICS)
 * List of Google Scopes: https://developers.google.com/identity/protocols/googlescopes
 * @return A google client object.
 */
function getServiceAccountClient() {
    try {   
        // Create and configure a new client object.        
        $client = new Google_Client();
        $client->useApplicationDefaultCredentials();
        $client->addScope([YOUR SCOPES HERE]);
        return $client;
    } catch (Exception $e) {
        print "An error occurred: " . $e->getMessage();
    }
}

function getOauth2Client() {
    try {

        $client = buildClient();

        // Set the refresh token on the client. 
        if (isset($_SESSION['refresh_token']) && $_SESSION['refresh_token']) {
            $client->refreshToken($_SESSION['refresh_token']);
        }

        // If the user has already authorized this app then get an access token
        // else redirect to ask the user to authorize access to Google Analytics.
        if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {

            // Set the access token on the client.
            $client->setAccessToken($_SESSION['access_token']);                 

            // Refresh the access token if it's expired.
            if ($client->isAccessTokenExpired()) {              
                $client->fetchAccessTokenWithRefreshToken($client->getRefreshToken());
                $client->setAccessToken($client->getAccessToken()); 
                $_SESSION['access_token'] = $client->getAccessToken();              
            }           
            return $client; 
        } else {
            // We do not have access request access.
            header('Location: ' . filter_var( $client->getRedirectUri(), FILTER_SANITIZE_URL));
        }
    } catch (Exception $e) {
        print "An error occurred: " . $e->getMessage();
    }
}