I don't have any experience with XOOPS2, but its best to try and strike out anything that can be causing probs. Do you know for sure if any other PHP scripts work on your site? you can check it by saving the following in a file and opening it online:
PHP Code:
<?php
phpinfo();
?>
That is also useful for diagnostics, like making sure paths are setup up correctly if you get other problems down the line.