We also have example source-code for PHP, which can generate the XML correctly, the previous example looks like:
<?php
require_once 'ma_rest_lib.inc.php';
$ma = new MA_REST_API();
$ma->add('download', array('filename' => 'test.txt'));
$ma->add('crm', array('email' => 'email@example.com', 'forename' => 'John', 'surname' => 'Smith'));
$ma->post();
?>