Gmail PHP API设计理解

I'm trying to get started with the Gmail PHP client library. I'm trying to understand the correlation between the language-specific Client libraries and the REST end-points. The API guid says nothing about all the methods available through the PHP Client library.

I want to read any incoming messages that contain a specific string

I found a few methods for example

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

// Print the labels in the user's account.
$user = 'me';
$results = $service->users_labels->listUsersLabels($user);

and

        $optParams = [];
    $optParams['maxResults'] = 5; // Return Only 5 Messages
    $optParams['labelIds'] = 'INBOX'; // Only show messages in Inbox
    $messages = $service->users_messages->listUsersMessages('me',$optParams);
    $list = $messages->getMessages();

But I just can't find any library methods to achieve the above goal. There can be some variations of https://www.googleapis.com/gmail/v1/users/me/messages?q="in:inbox"this method but that'll be going the REST end-point way.

• So is it like the REST end-point is the real deal and PHP client library is a helper for authentication but doesn't have all the functions that can be carried out with the REST API?
• IF not, what Client library methods can I use to get the incoming messages containing a specific string?