Creating a function to logout a WordPress user?

I’m trying to create a function to logout WordPress users after a period of inactivity. The timed element is working as it should and redirecting users after a set period of time.

The problem is that once the PHP script is loaded (in code sample), I run into the following error: Fatal error: Call to undefined function wp_logout() in /var/www/html/wp-content/plugins/ion-wp-login-timeout/scripts/timed-logout.php on line 5

All the reference material I read tells me that wp_logout() should log out the user but instead there are errors. I have tried several methods, including adding an action. I do not want to redirect the user to a login screen.

Sample of code is below. The echo statements are in place purely for testing. Any help would be much appreciated.

<?php

    function logout_this_session() {
        //Logout Now
        wp_logout();
        wp_die();
    }

    echo 'This will be the logout script<br/><br/>';

    $last_page = $_SERVER['HTTP_REFERER'];
    echo 'You came from: ' . $last_page;

    logout_this_session();

    header( 'Location: ' . $last_page );

?>

Answer

I finally figured out a solution as in code sample below. From the last stage of the process before logging out the user, I have a wpsessionexpired=true value posted back to the page the user was on. The same page is immediately refreshed after the user is logged out. I placed this within the main plugin file.

function logoutUser(){
    if ( $_POST["wpsessionexpired"] == 'true' ){ 
        wp_logout();
        header("refresh:0.5;url=".$_SERVER['REQUEST_URI']."");
    }
}
add_action('init', 'logoutUser');