<?php
use Microsoft\Graph\Beta\GraphServiceClient;
use Microsoft\Graph\Beta\Generated\Users\Item\Outlook\Tasks\Item\Complete\CompleteRequestBuilderPostRequestConfiguration;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestConfiguration = new CompleteRequestBuilderPostRequestConfiguration();
$headers = [
'Prefer' => 'outlook.timezone="Pacific Standard Time"',
];
$requestConfiguration->headers = $headers;
$result = $graphServiceClient->me()->outlook()->tasks()->byOutlookTaskId('outlookTask-id')->complete()->post($requestConfiguration)->wait();