The best solution may depend on the actual design/implementation of your app. If everything goes through a single index.php (or whatever) file, such as in a MVC sort of design, you could set a base path variable at the start of that file:
<?php
$basePath = dirname(__FILE__);
include $basePath . '/includes/foo/bar.php';
If the app is intended to always be installed in the root web directory where it is to be used, you can use $SERVER['DOCUMENT_ROOT']:
<?php
include $_SERVER['DOCUMENT_ROOT'] . '/includes/foo/bar.php';
Otherwise, you may need to set a variable or constant in a config file that gets require()'d by every script in the app:
<?php
define('BASE_DIR', 'C:/wampserver/www/my_app');
include BASE_DIR . '/includes/foo/bar.php';
Then your installation instructions (or program) would include the editing of that definition.