Template Tutorial

So I followed the tutorial and it made a lot of sense using a class etc so ive come up with this:

templateclass.php

<?php
class Page
{
var $page;
var $filename;
var $content;

//constructor function
function Page($template) {
if (file_exists($template)){
$this->page = join("", file($template));
}else{
die("Template file $template not found, please contact site administrator.");
}
}

//function to add title to page
function addToTitle($title) {
$this->page = str_replace("{title}", $title, $this->page);
}

//function to add scripts to page must be external scripts
function addScript($script) {
$this->page = str_replace("{script}", '<script type="text/javascript" src="'.$script.'"></script>', $this->page);
}

//function to add content to page
function addContent($content) {
ob_start();
include($content);
$contentData = ob_get_contents();
$this->page = str_replace("{content}", $contentData, $this->page);
ob_end_clean();
}

//function to add menu to page
function showMenu($menu) {
ob_start();
include($menu);
$menuData = ob_get_contents();
$this->page = str_replace("{mainmenu}", $menuData, $this->page);
ob_end_clean();
}


//Outputs final page code
function output() {
//if no script replace {script} with nothing
$this->page = str_replace("{script}", '', $this->page);
echo $this->page;
}
}
?>

about.php

<?
//start up
require_once('library/templateclass.php');

// Set up page titles
$page = new Page('library/template.php');
$page->addToTitle('About Us');
$page->showMenu('library/menu.php');
$page->addContent($content);
$page->output();

?>
<!-- Page content to replace {content} starts here-->
<h1>about us</h1>
<p>lorem ipsum dolor sit amet</p>

<?
//php code here
?>

Basically I want to be able to assign any content after <!-- page content.. --> to the $content variable to replace the {content} tag but got know idea how to approach this... (yes i know iinclude($content); wont work as $content is not a file)