<?php
use Microsoft\Graph\GraphServiceClient;
use Microsoft\Graph\Generated\ServicePrincipals\Item\AddPassword\AddPasswordPostRequestBody;
use Microsoft\Graph\Generated\Models\PasswordCredential;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new AddPasswordPostRequestBody();
$passwordCredential = new PasswordCredential();
$passwordCredential->setDisplayName('Password friendly name');
$requestBody->setPasswordCredential($passwordCredential);
$result = $graphServiceClient->servicePrincipals()->byServicePrincipalId('servicePrincipal-id')->addPassword()->post($requestBody)->wait();