<?php
use Microsoft\Graph\GraphServiceClient;
use Microsoft\Graph\Generated\Models\ConversationThread;
use Microsoft\Graph\Generated\Models\Post;
use Microsoft\Graph\Generated\Models\ItemBody;
use Microsoft\Graph\Generated\Models\BodyType;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new ConversationThread();
$requestBody->setTopic('Take your wellness days and rest');
$postsPost1 = new Post();
$postsPost1Body = new ItemBody();
$postsPost1Body->setContentType(new BodyType('html'));
$postsPost1Body->setContent('Waiting for the summer holidays.');
$postsPost1->setBody($postsPost1Body);
$postsArray []= $postsPost1;
$requestBody->setPosts($postsArray);
$result = $graphServiceClient->groups()->byGroupId('group-id')->conversations()->byConversationId('conversation-id')->threads()->post($requestBody)->wait();