Inclua o código abaixo no topo da página PHP.
//define constant
define( '_JEXEC', 1 );
define( 'DS', DIRECTORY_SEPARATOR );
// adjust joomla path according to your joomla installation
define( 'JPATH_BASE', $_SERVER[ 'DOCUMENT_ROOT' ] . DS . 'joomla' );
//include joomla core files
require_once( JPATH_BASE . DS . 'includes' . DS . 'defines.php' );
require_once( JPATH_BASE . DS . 'includes' . DS . 'framework.php' );
require_once( JPATH_BASE . DS . 'libraries' . DS . 'joomla' . DS . 'factory.php' );
//create application
$mainframe =& JFactory::getApplication('site');
//joomla functionality has been included!
//Now we initialize database object
$db = JFactory::getDBO();
//Importing the Plugin Helper
JLoader::import('joomla.plugin.helper');
// test whether some plugins is enabled
$pluginenabled = &JPluginHelper::isEnabled('authentication', 'joomla');