I wanted to use a a PHP include or require in a field in an RSForms Pro form. To do this properly, I needed the base URL of the site. Here's what worked to get the base URL and the rest of the include.
//<code>
require(preg_replace("!${_SERVER['SCRIPT_NAME']}$!", '', $_SERVER['SCRIPT_FILENAME']) . '/includes/testday.php');
$items=get_session_list();
return($items);
//</code>