How do I get back to a page and pass $_GET variables into it?

I have a login page that uses “form” $_POST variables and calls a second php file (non-displalyed page) to verify data entry. If the data is incorrect the second page passes back an error code to the parent page, a $_GET variable to display the appropriate error message on the form. Before wordpress I used the PHP header(“Location:” . URL . “?response_code=”) format. This doesn’t work in wordpress. Any ideas how to return to the parent page with the needed response code?

Thanks.

George