Google Calendar Server to Server

Question

I use to make appointments a calendar based on FULLCALENDAR with backup of the rdv in a database mysql on my own local server. So far, so good. I wish I could synchronize the rdvs thus taken, on the google calendar and inversely be able to import into my database mysql the rdvs taken on the account of google on line. I tried to understand and read the API doc of google calendar here and here I only get error messages. I tried to do it directly in fullcalendar but the calendar must be declared in public, something I can not do. The ideal would be to be able to make server calls in php (login and password of google clandier backup in local mysql database), for which I create my API key and a service account, but again I do not get only error messages. Would there be a link or a really functional tutorial that I could use to learn I tried with this link but never succeeded thanks in advance.

EDIT : I try this.

quickstart.php

<?php
require_once __DIR__ . '/vendor/autoload.php';


define('APPLICATION_NAME', 'Google Calendar API PHP Quickstart');
define('CREDENTIALS_PATH', '~/.credentials/calendar-php-quickstart.json');
define('CLIENT_SECRET_PATH', __DIR__ . '/client_id.json');
// If modifying these scopes, delete your previously saved credentials
// at ~/.credentials/calendar-php-quickstart.json
define('SCOPES', implode(' ', array(
  Google_Service_Calendar::CALENDAR_READONLY)
));

if (php_sapi_name() != 'cli') {
  throw 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 = new Google_Client();
  $client->setApplicationName(APPLICATION_NAME);
  $client->setScopes(SCOPES);
  $client->setAuthConfig(CLIENT_SECRET_PATH);
  $client->setAccessType('offline');

  // Load previously authorized credentials from a file.
  $credentialsPath = expandHomeDirectory(CREDENTIALS_PATH);
  if (file_exists($credentialsPath)) {
    $accessToken = json_decode(file_get_contents($credentialsPath), true);
  } 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);

    // Store the credentials to disk.
    if(!file_exists(dirname($credentialsPath))) {
      mkdir(dirname($credentialsPath), 0700, true);
    }
    file_put_contents($credentialsPath, json_encode($accessToken));
    printf("Credentials saved to %s\n", $credentialsPath);
  }
  $client->setAccessToken($accessToken);

  // Refresh the token if it's expired.
  if ($client->isAccessTokenExpired()) {
    $client->fetchAccessTokenWithRefreshToken($client->getRefreshToken());
    file_put_contents($credentialsPath, json_encode($client->getAccessToken()));
  }
  return $client;
}

/**
 * Expands the home directory alias '~' to the full path.
 * @param string $path the path to expand.
 * @return string the expanded path.
 */
function expandHomeDirectory($path) {
  $homeDirectory = getenv('HOME');
  if (empty($homeDirectory)) {
    $homeDirectory = getenv('HOMEDRIVE') . getenv('HOMEPATH');
  }
  return str_replace('~', realpath($homeDirectory), $path);
}

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

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

if (count($results->getItems()) == 0) {
  print "No upcoming events found.\n";
} else {
  print "Upcoming events:\n";
  foreach ($results->getItems() as $event) {
    $start = $event->start->dateTime;
    if (empty($start)) {
      $start = $event->start->date;
    }
    printf("%s (%s)\n", $event->getSummary(), $start);
  }
}

php quickstart.php

PHP Fatal error:  Uncaught InvalidArgumentException: missing the required redirect URI in /Users/krislec/Desktop/vendor/google/auth/src/OAuth2.php:648
Stack trace:
#0 /Users/krislec/Desktop/vendor/google/apiclient/src/Google/Client.php(340): Google\Auth\OAuth2->buildFullAuthorizationUri(Array)
#1 /Users/krislec/Desktop/quickstart.php(36): Google_Client->createAuthUrl()
#2 /Users/krislec/Desktop/quickstart.php(75): getClient()
#3 {main}
  thrown in /Users/krislec/Desktop/vendor/google/auth/src/OAuth2.php on line 648

Fatal error: Uncaught InvalidArgumentException: missing the required redirect URI in /Users/krislec/Desktop/vendor/google/auth/src/OAuth2.php:648
Stack trace:
#0 /Users/krislec/Desktop/vendor/google/apiclient/src/Google/Client.php(340): Google\Auth\OAuth2->buildFullAuthorizationUri(Array)
#1 /Users/krislec/Desktop/quickstart.php(36): Google_Client->createAuthUrl()
#2 /Users/krislec/Desktop/quickstart.php(75): getClient()
#3 {main}
  thrown in /Users/krislec/Desktop/vendor/google/auth/src/OAuth2.php on line 648