Minimal Drupal Bootstrap for AJAX requests

drupal-ajax.php

<?php

/**
 * @file
 *
 * Minimal AJAX page. Call like this:
 *  $.ajax({
 *       type: 'GET',
 *       url: '/drupal-ajax.php?param1=...
 *       data: {},
 *       dataType: 'json',
 *       success: function (data) {
 *         // Do stuff
 *       },
 *       error: function (xmlhttp) {
 *         alert(Drupal.t('An HTTP error @status occurred.', {'@status': xmlhttp.status}));
 *       }
 *     });
 */

define('DRUPAL_ROOT', getcwd());

// Include whatever you need. Base functions reside in Drupals' /includes folder
// You can also include module files, but be aware of dependencies!
require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
require_once DRUPAL_ROOT . '/includes/common.inc';
// ..

// If you want to use dd() for debugging purposes, include these too!
// require_once DRUPAL_ROOT . '/includes/file.inc';
// require_once DRUPAL_ROOT . '/sites/all/modules/contrib/devel/devel.module';

// Minimal bootstrap of Drupal
drupal_bootstrap(DRUPAL_BOOTSTRAP_SESSION);

// dd() uses stream wrappers in file_put_contents() to write drupal_debug.txt. 
// This function registers the implementations with PHP during the call.
// Uncomment too!
// file_get_stream_wrappers();

$output = '';

// Do logic!

// Set the correct content type
drupal_add_http_header('Content-Type', 'text/javascript; charset=utf-8');

// IE9 does a nasty client side caching. We explicitly trigger a cache miss
drupal_add_http_header('Cache-Control', 'no-store, no-cache, must-revalidate');
drupal_add_http_header('Pragma', 'no-cache');

// Return your output as a JSON object for easy JS integration
print drupal_json_encode(array(
  'content' => $output,
));

exit();

This does not work for me, I always get an empty string back from it despite the status being 200 (success).