function to render a topbar menu in Processwire using the bootstrap framework. Assumes you have a text field called "navbar_separator" which can be used to create a bootstrap separator element above the $page and textfield called "menu_title" which we use to create an override of the $page->title to customize the menu.

renderResponsiveNavbar.php

<?php 
/**
 * renderResponsiveNavbar function for Processwire CMS.
 * 
 * Author - Jeff Selser 
 * 01/20/2013
 * @access public
 * @param mixed $page
 * @param string $brand (default: SProject Name')
 * @param int $maxChilden (default: 15) sets limit on number of children in a dropdown
 * @param int $levels (default: 2)
 * @return void
 * @TODO - Add support for three levels (2 is max now)
 */

function renderResponsiveNavbar($page, $brand='Project Name', $levels=2, $maxChildren=15){

  $homepage = wire('pages')->get('/');
	$cache = wire('modules')->get('MarkupCache'); 
	// check the cache if not current (1day) build the menu, when testing set it to 0
	if(!$navbar = $cache->get('navbar', 86400)){
	
	// build the bootstrap html
	$navbar = "<div class='navbar navbar-inverse'>\n";
	$navbar .= "\t<div class='navbar-inner'>\n";
	$navbar .= "\t\t<div class='container'>\n";
	$navbar .= "\t\t\t<a class='btn btn-navbar' data-toggle='collapse' data-target='.nav-collapse'>\n";
	$navbar .= "\t\t\t\t<span class='icon-bar'></span>\n";
	$navbar .= "<span class='icon-bar'></span>\n";
	$navbar .= "<span class='icon-bar'></span>\n";
	$navbar .= "\t\t\t\t</a>";
	
	// display the brand html if set
	if ($brand) {
		
		$navbar .= "<a class='brand visible-phone' href='/'>{$brand}</a>\n";
	}
	
	$navbar .= "<div class='nav-collapse collapse'>\n";
		
	$ul = "<ul class='nav'>\n";
	 
	 // loop through the children of the parent meny items. 
	foreach($homepage->children as $child) {
		if($levels == 2 && $child->id > 1 && $child->numChildren && $child->numChildren < $maxChildren) {
				
				// add  data-target='{$child->url}' to anchor to click target
				
				$li .= 	"\t<li class='dropdown'>" . 
				"<a href='{$child->url}' class='dropdown-toggle' data-toggle='dropdown' data-target='{$child->url}'>{$child->get("menu_title|title")}<b class='caret'></b></a>"; 

					$li .= "\n\t\t<ul class='dropdown-menu'>\n";
	
					foreach($child->children() as $c) {
						
						if($c->navbar_separator){
	
								$li .= "<li class='divider'></li>\n" .
									   "<li class='nav-header'>{$c->navbar_separator}</li>";
						
						}					
					
						$li .= "\t\t\t<li><a href='{$c->url}'>{$c->get("menu_title|title")}</a></li>\n";
					}

					$li .= "</ul>";

		} else {
				
				$class = $child === $page->rootParent ? " class='active'" : '';

				$li .= 	"\n\t<li$class>" . 
						"<a href='{$child->url}'>{$child->get("menu_title|title")}</a>";
	
		}
		
	}
	
		
	$ul .= $li;
	$ul .= "</ul>\n";

	// pull a contact link to the right
	
	$navbar .= $ul;	
	$navbar .= "<ul class='nav pull-right'>\n";
	$navbar .= "\t\t<li><a href='/contact/'>Contact</a></li>\n";
	$navbar .= "</ul>\n";
	
	// Close out the divs	
	$navbar .= "</div>\n</div>\n</div>\n</div>";
	
	}
	
	// save the menu to the cache (if not the same)
	$cache->save($navbar);
	
	// return the HTML	
	return $navbar;
	
}