My problem was I didnt want php code in my views footer. But I still needed to check if a module existed to call a function of that module.
Here is the hook_views_pre_render, what happens in the function is :
- checking for the correct view and adding content to the footer
- setting the filter format which is the full html id
- setting the empty option

With this function not only the footer could be altered but the entire view could be altered since the views object is present.

/**
 * Implementation of hook_views_pre_render()
 */
function wishlist_views_pre_render(&$view) {
  if($view->name == 'wishlist') {
    if (module_exists('orderlist')) {
      $view->display_handler->set_option('footer', orderlist_admin_settings());
      $view->display_handler->set_option('footer_format', '2');
      $view->display_handler->set_option('footer_empty', 0);
    }
  }
}

For more http://drupalcontrib.org/api/function/hook_views_pre_render/6