CakePHP1.3_Smarty3_Themeing.php

<?php
/**
 * A custom view class that is used for themeing
 *
 * PHP versions 4 and 5
 *
 * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
 * Copyright 2005-2010, Cake Software Foundation, Inc. (http://cakefoundation.org)
 *
 * Licensed under The MIT License
 * Redistributions of files must retain the above copyright notice.
 *
 * @copyright     Copyright 2005-2010, Cake Software Foundation, Inc. (http://cakefoundation.org)
 * @link          http://cakephp.org CakePHP(tm) Project
 * @package       cake
 * @subpackage    cake.cake.libs.view
 * @since         CakePHP(tm) v 0.10.0.1076
 * @license       MIT License (http://www.opensource.org/licenses/mit-license.php)
 */

/**
 * Theme view class
 *
 * Allows the creation of multiple themes to be used in an app. Theme views are regular view files
 * that can provide unique HTML and static assets.  If theme views are not found for the current view
 * the default app view files will be used. You can set `$this->theme` and `$this->view = 'Theme'` 
 * in your Controller to use the ThemeView.
 *
 * Example of theme path with `$this->theme = 'super_hot';` Would be `app/views/themed/super_hot/posts`
 *
 * @package       cake
 * @subpackage    cake.cake.libs.view
 */
class ThemeView extends View {
     public $layout = "layout";
     public $ext = ".html";

/**
 * Constructor for ThemeView sets $this->theme.
 *
 * @param Controller $controller
 */
	function __construct(&$controller, $register = true) {
		parent::__construct($controller, $register);
		$this->theme =& $controller->theme;
             App::import("Vendor", "Smarty", array('file' => 'smarty'.DS.'Smarty.class.php'));
             $this->Smarty = &new Smarty();
             $this->Smarty->plugins_dir[] = VIEWS.'smarty_plugins'.DS;
             $this->Smarty->compile_dir = TMP.'smarty'.DS.'compile'.DS;
             $this->Smarty->cache_dir = TMP.'smarty'.DS.'cache'.DS;
             $this->Smarty->error_reporting = 'E_ALL | E_STRICT';
             $this->Smarty->debugging = true;

          $this->viewVars['params'] = $this->params;
	}

     function &_loadHelpers(&$loaded, $helpers, $parent = null)
     {
          $loaded = parent::_loadHelpers($loaded, $helpers, $parent);

          if( !$parent ) {
               $prefix = Configure::read("ext_prefix");
               foreach(array_keys($loaded) as $helper) {
                    if( strpos($helper, $prefix) !== false) {
                         $loaded[str_replace($prefix, "", $helper)] = $loaded[$helper];
                         unset($loaded[$helper]);
                    }
               }
          }

          return $loaded;
     }

/**
 * Renders and returns output for given view filename with its
 * array of data.
 *
 * @param string $___viewFn Filename of the view
 * @param array $___dataForView Data to include in rendered view
 * @param boolean $loadHelpers Boolean to indicate that helpers should be loaded.
 * @param boolean $cached Whether or not to trigger the creation of a cache file.
 * @return string Rendered output
 * @access protected
 */
	function _render($___viewFn, $___dataForView, $loadHelpers = true, $cached = false) {
		$loadedHelpers = array();

		if ($this->helpers != false && $loadHelpers === true) {
			$loadedHelpers = $this->_loadHelpers($loadedHelpers, $this->helpers);
			$helpers = array_keys($loadedHelpers);
			$helperNames = array_map(array('Inflector', 'variable'), $helpers);

			for ($i = count($helpers) - 1; $i >= 0; $i--) {
				$name = $helperNames[$i];
				$helper =& $loadedHelpers[$helpers[$i]];

				if (!isset($___dataForView[$name])) {
					${$name} =& $helper;
				}
				$this->loaded[$helperNames[$i]] =& $helper;
                    $this->Smarty->assignByRef($name, $helper);
			}
			$this->_triggerHelpers('beforeRender');
			unset($name, $loadedHelpers, $helpers, $i, $helperNames, $helper);
		}
          $this->Smarty->assign($___dataForView);
        $this->Smarty->assignByRef('view', $this);
        return $this->Smarty->fetch($___viewFn);
	}
}

Why the leading underscores on $___viewFn and $___dataForView?

That is how they are written in the original Cake view file class. No special reason other than that! :)