<?php
use Microsoft\Graph\GraphServiceClient;
use Microsoft\Graph\Generated\Models\UnifiedRoleAssignment;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new UnifiedRoleAssignment();
$requestBody->setOdataType('#microsoft.graph.unifiedRoleAssignment');
$requestBody->setRoleDefinitionId('c2cf284d-6c41-4e6b-afac-4b80928c9034');
$requestBody->setPrincipalId('f8ca5a85-489a-49a0-b555-0a6d81e56f0d');
$requestBody->setDirectoryScopeId('/');
$result = $graphServiceClient->roleManagement()->directory()->roleAssignments()->post($requestBody)->wait();