我的谷歌php客户端刷新令牌为空

Question

我正在尝试使用Google_Client类获取用于身份验证的刷新令牌。这就是我想要做的。 使用包"google/apiclient": "^2.2",

$client = new \Google_Client($config);
$client->setAccessType("offline"); 
$client->setState('offline');
$client->setApprovalPrompt('force');
$client->setIncludeGrantedScopes(true);
var_dump($client->authenticate('<auth_token_goes_here>');); // dumps array which has id_token, access_token , expires_in, token_type & created
var_dump($googleClient->getAccessToken()); // dumps array which has id_token, access_token , expires_in, token_type & created
var_dump($googleClient->getRefreshToken()); // returns null

这里有什么问题？