无法从google api OAuth2中刷新令牌

这是我的php脚本RefreshToken.php

<?php
$url = 'https://accounts.google.com/o/oauth2/token';
$post_data = array(
                    'code'          =>   'xxxxxxxx',
                    'client_id'     =>   'xxxxxxxxxxx',
                    'client_secret' =>   'xxxxxxxxxxxxx',
                    'redirect_uri'  =>   'http://localhost/googleapi/AuthenticationCode.php',
                    'grant_type'    =>   'authorization_code',
                    );
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$result = curl_exec($ch);
$token = json_decode($result);
echo $token->refresh_token . "'n";
?>

运行PHP CLI

php -q RefreshToken.php

PHP注意:未定义属性:stdClass::$refresh_token在/var/www/googleapi/refreshtoken . PHP第20行