a quite simple helloworld webapp with joomla platform 12.1

index.php

<?php
// We are a valid Joomla entry point.
define('_JEXEC', 1);

// Setup the base path related constant.
define('JPATH_BASE', dirname(__FILE__));
define('JPATH_SITE', JPATH_BASE);
define('JPATH_LIBRARIES', dirname(dirname(__FILE__)) . '/joomla/libraries');

require JPATH_LIBRARIES.'/import.php';


class Hello extends JApplicationWeb
{
	protected function doExecute()
	{
		$this->setBody('Hello World!');
	}

}

$app = JApplicationWeb::getInstance('Hello');
JFactory::$application = $app;

$app->execute();