Include Joomla core functionality in external PHP file

<?php
//define constant
define( '_JEXEC', 1 );
define( 'DS', DIRECTORY_SEPARATOR );
 
//you need to 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');
 
//in this point, you're ready to use joomla functionality
 
//example: initialize database object
$db = JFactory::getDBO();
 
//or import plugin helper
JLoader::import('joomla.plugin.helper');
 
//to test whether some plugins is enabled
$pluginenabled = &JPluginHelper::isEnabled('authentication', 'joomla');