I would like to resolve the output of "error": "invalid_grant" in Google Calendar API.

PHP Fatal error:Uncaught Google\Service\Exception: {
  "error": "invalid_grant",
  "error_description": "Token has been expired or revoked."
}

require__DIR__.'/vendor/autoload.php';
define('CLIENT_PATH',__DIR__.'/client.json');
if(php_sapi_name()!='cli'){
    through new Exception('This application must be run on the command line.');
}

/**
 * Returns an authorized API client.
 * @return Google_Client the authorized client object
 */
    function getClient()
{
    $client=newGoogle_Client();
    $client->setApplicationName('Google Calendar API PHP Quickstart');
    $client->setScopes(Google_Service_Calendar::CALENDAR_EVENTS);
    $client->setAuthConfig(CLIENT_PATH);
    $client->setAccessType('offline');
    $client->setPrompt('select_account content');

    // Load previously authorized token from a file, if it exists.
    // The file token.json stores the user's access and refresh tokens, and is
    // created automatically when the authorization flow complements for the first
    // time
    $tokenPath='token.json';
    if(file_exists($tokenPath)){
        $accessToken=json_decode(file_get_contents($tokenPath), true);
        $client->setAccessToken($accessToken);
    }

    // If there is no previous token or it's expired.
    if($client->isAccessTokenExpired()){
        // Refresh the token if possible, else fetch a new one.
        if($client->getRefreshToken()){
            $client->fetchAccessTokenWithRefreshToken($client->getRefreshToken());
        } else{
            // Request authorization from the user.
            $authUrl=$client->createAuthUrl();
            printf("Open the following link in your browser:\n%s\n", $authUrl);
            print 'Enter verification code:';
            $authCode=trim(fgets(STDIN)));

            // Exchange authorization code for an access token.
            $accessToken=$client-> fetchAccessTokenWithAuthCode($authCode);
            $client->setAccessToken($accessToken);

            // Check to see if there was an error.
            if(array_key_exists('error',$accessToken'){
                through new Exception (join(',',',$accessToken);
            }
        }
        // Save the token to a file.
        if(!file_exists(dirname($tokenPath))))){
            mkdir(dirname($tokenPath), 0700, true);
        }
        file_put_contents($tokenPath, json_encode($client->getAccessToken()));
    }
    return$client;
}

// Get the API client and construct the service object.
$client=getClient();
$service=newGoogle_Service_Calendar($client);

// Print the next 10 events on the user's calendar.
$calendarId='primary';
$optParams=array(
    'maxResults' = > 15,
    'orderBy' = > 'startTime',
    'singleEvents' = > true,
    'timeMin' = > date('c'),
);
$results=$service->events->listEvents($calendarId,$optParams);
$events=$results->getItems();

if(empty($events)){
    print "No upcoming events found.\n";
} else{
    print "Upcoming events:\n";
    foreach($events as$event){
        $start = $event->start->dateTime;
        if(empty($start)){
            $start = $event->start->date;
        }
        printf("%s(%s)\n", $event->getSummary(), $start);
    }
}