page_template toggle between 2 templates + 2 permalinks for same post

Question

I'm trying to use this page_template template from the WP forums, with the same aim as the original author:

I am trying to use two separate permalinks/templates for a single post - each providing different views of the post (through different templates). http://example.com/category/post-name -> uses template1.php http://example.com/category/post-name/details -> uses template2.php

I've modified the code according to the changes discussed in the thread, but my alternative template (for a custom post type) isn't being loaded — /post-name/detail just redirects to /post-name/.

Any ideas?

// Add 'detail' to the permalink structure / query string 
function detail_rewrite_rules( $rules ) {
    $newrules = array();
    $newrules['(.+?)/([^/]+)(/[0-9]+)?/detail/?$'] = 'index.php?category_name=$matches[1]&name=$matches[2]&page=$matches[3]&detail=1'; 

    return $newrules + $rules;
}
add_filter( 'rewrite_rules_array','detail_rewrite_rules' );

function detail_flush_rules(){
    $rules = get_option( 'rewrite_rules' );

    if ( ! isset( $rules['(.+?)/([^/]+)(/[0-9]+)?/detail/?$'] )   ) {

        global $wp_rewrite;
        $wp_rewrite->flush_rules();
    }
}
add_action( 'init','detail_flush_rules' );

// Open up 'detail'
function detail_queryvars( $qvars ) {
  $qvars[] = 'detail';
  return $qvars;
}
add_filter('query_vars', 'detail_queryvars' );

// Switch between templates
function filter_single_template($template){

    $object = get_queried_object();
    $templates = array();

    /* If detail is in the query string, create a list of templates to use */
    // if($_GET['detail']) {
    // if( get_query_var('detail') ) {
    global $wp_query;
    if ( $wp_query->query_vars['detail'] ) {    
        echo "<script>console.log('Detail template activated');</script>";
        $templates[] = "single-photos-detail.php";
        // $templates[] = "detail-single.php";
        // $templates[] = "detail-single-{$object->post_type}.php";
    }

    /* If one of our custom detail templates exists, return it.
     * Otherwise return the original template
     */
    $templateSwitch = locate_template($templates);
    return (!empty($templateSwitch)) ? locate_template($templates) : $template;
}
add_filter('page_template', 'filter_single_template');

Answer 1

WordPress has a simpler function for what you're trying to do, add_rewrite_endpoint. Additionally, the filter for a single post template is single_template, page_template fires on the page post type.

function wpd_detail_endpoint(){
    add_rewrite_endpoint( 'detail', EP_PERMALINK );
}
add_action( 'init', 'wpd_detail_endpoint' );

function wpd_detail_template( $template = '' ) {
    global $wp_query;
    if( ! array_key_exists( 'detail', $wp_query->query_vars ) ) return $template;

    $template = locate_template( 'single-photos-detail.php' );
    return $template;
}
add_filter( 'single_template', 'wpd_detail_template' );

This assumes the template exists, you may want to do some additional checks as you've got in your original function.