<?php
use Microsoft\Graph\GraphServiceClient;
use Microsoft\Graph\Generated\Models\AppRoleAssignment;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new AppRoleAssignment();
$requestBody->setPrincipalId('cde330e5-2150-4c11-9c5b-14bfdc948c79');
$requestBody->setResourceId('8e881353-1735-45af-af21-ee1344582a4d');
$requestBody->setAppRoleId('00000000-0000-0000-0000-000000000000');
$result = $graphServiceClient->users()->byUserId('user-id')->appRoleAssignments()->post($requestBody)->wait();