guzzleproxy.php

<?php

$http = new GuzzleHttp\Client;

$response = $http->post(url('oauth/token'), [
    'form_params' => [
        'grant_type' => 'password',
        'client_id' => 2,
        'client_secret' => 'SECRET_HERE',
        'username' => 'user@example.com',
        'password' => 'supersecret',
        'scope' => ''
    ]
]);