undefined function get_header() after submit form

Hi there –
I’m migrating an existing static site into wordpress for the first time, so please be kind.

We use a custom form so I haven’t researched any plugins, and I’d love to get this to work as-is.

The problem is that after the form is submitted and validates, I need the page to redirect to a page with a thank you message, and it’s throwing a fatal error instead.

The form action is set to send the form to an external validation file. The vaildation file then redirects the browser to:

header ('Location: involved_thanks_volunteer.php');

and then the error message loads:

Fatal error: Call to undefined function get_header() in /home/panmac11/phoenixrisingtheater.org/dynamic/wp-content/themes/phoenixtheme/involved_thanks_volunteer.php on line 11

Even though the form validates and then is emailed properly.

I’ve set up involved_thanks_volunteer.php as a page template, as was suggested in this forum elsewhere, but the same error occurs.

Here’s the top of my code for involved_thanks_volunteer.php”

<?php
/* Template Name: Thank You Volunteer Form 

*/

?>

<?php get_header(); ?>  

<?php include(TEMPLATEPATH . '/inc/nav.php'); ?>

and then the content.

Any help would be appreciated. You can see the form here:
https://www.phoenixrisingtheater.org/dynamic/get-involved/