token.php

<?php

// given a proper $token and $consumer...

require_once('include/api/RestService.php');
require_once('clients/base/api/OAuth2Api.php');

$service = new RestService();
$api = new OAuth2Api();

$response = $api->token($service,array(
    "client_secret" => $consumer->c_secret,
    "client_id" => $consumer->c_key,
    "platform" => $token->platform,
    "grant_type" => $grant_type,
    "refresh_token" => $token->id,
));

// ^^^ throws exception in the logs:
// Exception in Controller: {"error":"invalid_request","error_description":"invalid_request"}