Wordpress - Rewrite from ?demo to /demo/

Question

Problem

I want to rewrite my GET-variable to /demo/.

My current permalink structure:
/my-new-post/?demo=true

What I want:
/my-new-post/demo/

I have this code:

It allows the "demo" $_GET variable and then echo "true". It works, just the rewrite part left.

<?php
add_filter('query_vars', 'queryvars' );
add_action('parse_query', 'echo_query_var');

function queryvars( $qvars )
{
    $qvars[] = 'demo';
    return $qvars;
}

function echo_query_var() {
    echo get_query_var('demo');
}
?>

Additional information

• It should work with all posts (not just "my-new-post").
• It should not return 404.
• I should be able to get the post ID.
• GET variable "demo" can be true, or empty. Not important.

Answer 1

This works so far...

// Actions - Rewrite rules
add_filter('query_vars', array($wp_comment_pages, 'add_get_variable'));
add_action('generate_rewrite_rules', array($wp_comment_pages, 'add_rewrite_rule'));

// Plugin container
class wp_comment_pages
{   
    // Flush the rules - only needed once
    function flush_rules()
    {
        global $wp_rewrite;
        $wp_rewrite->flush_rules();
    }

    // Add get variable to query vars
    function add_get_variable($public_query_vars)
    {
        $public_query_vars[] = 'demo';
        return $public_query_vars;
    }

    // Generate permalinks
    function add_rewrite_rule($wp_rewrite)
    {
        $new_rules = array(
            '(.+)/demo' => 'index.php?p=' . $wp_rewrite->preg_index(1) . '&demo=true'
        );

        $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
    }
}