I searched a long night and this is doing the trick:
Modified user-example.php from admin-sdk
$client->setAccessType('offline');
$client->setApprovalPrompt('force');
$authUrl = $client->createAuthUrl();
echo "<a class='login' href='" . $authUrl . "'>Connect Me!</a>";
then you get the code at the redirect url and the authenticating with the code and getting the refresh token
$client()->authenticate($_GET['code']);
echo $client()->getRefreshToken();
You should store it now ;)
When your accesskey times out just do
$client->refreshToken($theRefreshTokenYouHadStored);