Dreamweaver will use the virtual command if you set the option in your site definition to create links relative to the site.
If you change this to create links relative to the document, it will use the require once command instead.
To trouble shoot the problem, try adding the following code at line 1 of your page to turn error reporting on:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>