How to use Handlebars in Mandrill PHP

mandril.php

<?php

//This API https://bitbucket.org/mailchimp/mandrill-api-php
require_once 'mandrill-api-php/src/Mandrill.php';

function send($data) {
    try {
        $mandrill = new Mandrill('YOUR_API_KEY');

        $message['to'][] = array(
            'email' => $data['email'],
            'name'  => $data['name'],
            'type'  => 'to'
        );

        $message['auto_text']      = true;
        $message['inline_css']     = true;
        $message['merge']          = true;
        $message['merge_language'] = 'handlebars';

        $merge_vars = array(
            'rcpt' => $data['email'],
            'vars' => array()
        );

        $merge_vars['vars'][] = array(
            'name' => 'name',
            'content' => $data['name']
        );

        $merge_vars['vars'][] = array(
            'name' => 'custom_text',
            'content' => $data['custom_text']
        );

        $message['merge_vars'][] = $merge_vars;

        $message['X-MC-MergeLanguage'] = json_encode(array(
            'message' => $message
        ));

        $mandrill
            ->messages
            ->sendTemplate('YOUR_TEMPLATE_NAME', null, $message, false);

        return "ok";

    } catch(Exception $e) {
        return $e->getMessage();
    }
}

$data = array(
    'name'        => 'Jhon Foo',
    'email'       => 'foo@bar.com',
    'custom_text' => 'anda a laar!'
);

print send($data);

$message['X-MC-MergeLanguage'] serves no purpose that I can see.