Example WFCustomFileSystem extension

gistfile1.php

<?php
/**
 * @package   	JCE
 * @copyright 	Copyright (c) 2009-2015 Ryan Demmer. All rights reserved.
 * @license   	GNU/GPL 2 or later - http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 * JCE is free software. This version may have been modified pursuant
 * to the GNU General Public License, and as distributed it includes or
 * is derivative of works licensed under the GNU General Public License or
 * other free or open source software licenses.
 */
defined('_JEXEC') or die('RESTRICTED');

require_once(__DIR__ . '/joomla.php');

class WFCustomFileSystem extends WFJoomlaSystem {
	public function getRootDir() {
		static $root;
	        
	        if (!isset($root)) {
	           // Get base directory as shared parameter
            	   $dir = $this->get('dir', '');
            	   
            	   // get custom variable to replace
            	   $custom 	= JRequest::getVar('custom_var', '');
            	   // replace value
            	   $dir 	= str_replace('$custom', $custom, $dir);
            	   
            	   // set dir variable with custom
            	   $this->set('dir', $dir);
            	   
            	   // use parent class to process root directory
            	   $root = parent::getRootDir();
	        }
	        return $root;
	}
}