index.php

<?php
/**
 * This file grabs requested html files.
 *
 * @author Paul Vorbach <vorbach@genitis.org>
 * @license http://opensource.org/licenses/mit-license.php MIT License
 * @version 0.1.0
 * @package org.genitis.yuki
 */

// Set <code>$_GET['q']</code> as <code>$path</code>, default is
// <code>''</code>.
$path = '';
if (isset($_GET['q'])) {
	$path = $_GET['q'];
	unset($_GET['q']);
}

// Set <code>$content</code> and <code>$index</code>
if ($path == '') {
	$content = '.';
	$index = 'index';
} elseif (strrpos($path, 'index/')) {
	// Redirect to a 404 error, no content file has been found
	require_once '../lib/functions.php';
	redirect(404, 'error/404/', $path);
} else {
	$content = $path;
	$index = $content.'index';
	$content = rtrim($content, '/'); // remove trailing slash
}

$content_path = $content.'.html';
$index_path = $index.'.html';

// Try to include the file <em>path/to/file.html</em> first.
// Then try to include <em>path/to/file/index.html</em>.
if (file_exists($content_path)) {
	include $content_path;
	exit;
} elseif (file_exists($index_path)) {
	include $index_path;
	exit;
}

// Redirections
include '../lib/redirections.php';
require_once '../lib/functions.php';
if (isset($redirections[$path]))
	redirect(301, $redirections[$path]);

// Redirect to a 404 error, no content file has been found
redirect(404, 'error/404/', $path);

Removed some parts for the sake of simplicity.

Between the include $…_path; and exit; statements you may save back your page visits or something like that.