Ticket #19077: custom-post-type-sub-page-test.php

<?php
/*
Plugin Name: Custom Post Type Sub-Page Test Plugin
Version: 1.0
*/

add_action('init', 'test_init');

function test_init() {
  register_post_type('mycustomposttype', array(
    'label' => 'My Custom Post Type',
    'show_ui' => true
  ));

  add_action('admin_menu', 'test_adminmenu');
}

function test_adminmenu() {
  add_submenu_page(
    'edit.php?post_type=mycustomposttype',
    'My Custom Post Type Sub-Page',
    'Test Sub-Page',
    'read',
    'customposttypesubpage',
    'test_subpage'
  );
}

function test_subpage() {
  echo 'Hello!';
}
?>