Add Extra Parameter in Single Post

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Endpoint Example
 * Description: Adds a permalink endpoint to posts named <code>epex</code>
 */

add_action( 'init', 't5_add_epex' );

function t5_add_epex()
{
    add_rewrite_endpoint( 'epex', EP_PERMALINK );
}

add_action( 'template_redirect', 't5_render_epex' );

/**
 * Handle calls to the endpoint.
 */
function t5_render_epex()
{
    if ( ! is_singular() or ! get_query_var( 'epex' ) )
    {
        return;
    }

    // You will probably do something more productive.
    $post = get_queried_object();
    print '<pre>' . htmlspecialchars( print_r( $post, TRUE ) ) . '</pre>';
    exit;
}


add_filter( 'request', 't5_set_epex_var' );

/**
 * Make sure that 'get_query_var( 'epex' )' will not return just an empty string if it is set.
 *
 * @param  array $vars
 * @return array
 */
function t5_set_epex_var( $vars )
{
    isset( $vars['epex'] ) and $vars['epex'] = true;
    return $vars;
}