get_auth_code.php

<?php


if (!file_exists(__DIR__ . '/vendor/autoload.php')) {
  throw new Exception(sprintf('Please run "composer require google/apiclient:~2.0" in "%s"', __DIR__));
}
require_once __DIR__ . '/vendor/autoload.php';

$client = new Google_Client();
$client->setApplicationName('');
$client->setScopes([
    'https://www.googleapis.com/auth/youtube',
]);

$client->setAuthConfig(__DIR__ . '/client_secret.json');
$client->setAccessType('offline');

// Request authorization from the user.
$authUrl = $client->createAuthUrl();
printf("Open this link in your browser:\n%s\n", $authUrl);