Url Links not working properly

[Goshawk]

Hi Team,

I am trying to customise a zencart and have come accross an unusual problem:

My url on my links are showing up as:
http://www.my_domain.com/folder/www.my_domain.com/folder/indexphp?main_page=contact_us

My configure.php has:

  define('HTTP_SERVER', 'www.my_domain.com');
  define('HTTPS_SERVER', 'www.my_domain.com');
  define('DIR_WS_PLANNER', '/folder/');
  define('DIR_WS_HTTPS_PLANNER', '/folder/'); 

and my links in the header:

<li><a href="<?php echo zen_href_link(FILENAME_CONTACT_US, '', 'SSL'); ?>"><?php echo HEADER_TITLE_CONTACT_US; ?></a></li>

zen_href_link in html_output_php:

function zen_href_link($page = '', $parameters = '', $connection = 'NONSSL', $add_session_id = true, $search_engine_safe = true, $static = false, $use_dir_ws_planner = true) {
    global $request_type, $session_started, $http_domain, $https_domain; 

Where would it duplicate part of the URL?
Where else should I look, is there a function I could have altered?