CPT, WP/CP | Type: PHPCreate sub posts for a WordPress custom post type

This has to do with creating hierarchical CPTs, so you can have ‘sub-posts’ as children of a CPT single.

// Fake pages' permalinks and titles. Change these to your required sub pages.
$my_fake_pages = array(
    'reviews' => 'Reviews',
    'purchase' => 'Purchase',
    'author' => 'Author Bio'
);

add_filter('rewrite_rules_array', 'fsp_insertrules');
add_filter('query_vars', 'fsp_insertqv');

// Adding fake pages' rewrite rules
function wpse_261271_insertrules($rules)
{
    global $my_fake_pages;

    $newrules = array();
    foreach ($my_fake_pages as $slug => $title)
        $newrules['books/([^/]+)/' . $slug . '/?$'] = 'index.php?books=$matches[1]&fpage=' . $slug;

    return $newrules + $rules;
}

// Tell WordPress to accept our custom query variable
function wpse_261271_insertqv($vars)
{
    array_push($vars, 'fpage');
    return $vars;
}

// Remove WordPress's default canonical handling function

remove_filter('wp_head', 'rel_canonical');
add_filter('wp_head', 'fsp_rel_canonical');
function wpse_261271_rel_canonical()
{
    global $current_fp, $wp_the_query;

    if (!is_singular())
        return;

    if (!$id = $wp_the_query->get_queried_object_id())
        return;

    $link = trailingslashit(get_permalink($id));

    // Make sure fake pages' permalinks are canonical
    if (!empty($current_fp))
        $link .= user_trailingslashit($current_fp);

    echo '<link rel="canonical" href="'.$link.'" />';
}