Add Custom Post Type to WordPress Plugin Submenu (Hack)

cpt_in_wp_submenu.php

<?php

/*
	Plugin Name: Nerfherder Plugin
	Plugin URI: https://gist.github.com/johnregan3/6133389
	Description: Demonstration of how to add Custom Post Types to Submenu Items
	Author: John Regan
	Author URI: http://johnregan3.com
	Version: 1.0
 */


/**
 * Create a post type ("Nerfherders") and add it to the Submenu Page under a Plugin's top-level Menu Page on the WordPress Dashboard.
 *
 * The main part of this technique is using redirects to send users to the Custom Post Type screens, while hiding the CPT from the main menu.
 *
 * Save this as a PHP file, maybe named "nerferder-plugin.php" and put it in a folder with the same name.
 * Upload it to your wp-content/plugins directory and ativate the Plugin to see it in action.
 */


/**
 * Set up Custom Post Type "Nerfherders."
 * Nothing significant here.
 */
add_action( 'init', 'create_post_type' );

function create_post_type() {
	$labels = array(
		'name'               => _x( 'Nerfherders', 'Plural Name' ),
		'singular_name'      => _x( 'Nerfherder', 'Singular Name' ),
		'add_new'            => _x( 'Add New', 'nerfherder' ),
		'add_new_item'       => __( 'Add New Nerfherder' ),
		'edit_item'          => __( 'Edit Nerfherder' ),
		'new_item'           => __( 'New Nerfherder' ),
		'all_items'          => __( 'All Nerfherders' ),
		'view_item'          => __( 'View Nerfherder' ),
		'search_items'       => __( 'Search Nerfherders' ),
		'not_found'          => __( 'No Nerfherders found' ),
		'not_found_in_trash' => __( 'No Nerfherders found in the Trash' ),
		'parent_item_colon'  => '',
		'menu_name'          => 'Nerfherders',
	);
	$args = array(
		'labels'      => $labels,
		'public'      => true,
		'has_archive' => true,
	);
	register_post_type( 'nerfherder_post_type', $args );
}

/**
 * Normally, this would appear as a top-level "Menu Page" in the WordPress Admin
 * We're going to reposition it underneath our in our Menu Page's ("Nerferder Plugin") Sub Menu Page
 */

/**
 * Create the top-level Admin Menu Page "Nerfherder Plugin"
 * Nothing significant here.
 */
add_action( 'admin_menu', 'register_nerfherder_menu_page' );

function register_nerfherder_menu_page(){
	add_menu_page( __( 'Nerfherder Plugin', 'nerfherder' ), __( 'Nerfherder Plugin', 'nerfherder' ), 'manage_options', basename(__FILE__), 'nerfherder_render_menu_page' );
}

function nerfherder_render_menu_page(){
	?>
		<div class="wrap">
			<div class="icon32" id="icon-options-general">
				<br />
			</div>
			<?php _e( '<h2>Nerfherder Plugin Settings</h2>', 'gamwp'); ?>
			<p>Click on one of the links in the "Nerfherder Plugin" Submenu to see the Custom Post Type Listing/Edit screens.</p>
		</div>
	<?php
}

/**
 * Create the Submenu items containing redirects to the Custom Post Type Pages
 * This is where the magic happens.
 */

//Add To Menu
add_action( 'admin_menu', 'nerfherder_add_submenu_pages' );

function nerfherder_add_submenu_pages() {
	add_submenu_page( 'nerfherder-plugin.php', __( 'All Nerfherders', 'nerfherders' ), __( 'All Nerfherders', 'nerfherders' ), 'manage_options', 'nerfherder_plugin_show_posts', 'nerfherder_render_nerferhders' );
	add_submenu_page( 'nerfherder-plugin.php', __( 'Add New Nerfherder', 'nerfherders' ), __( 'Add New Nerfherder', 'nerfherders' ), 'manage_options', 'nerfherder_plugin_add_post', 'nerfherder_render_new_nerferhders' );

}

function nerfherder_render_nerferhders(){
	$url = admin_url().'edit.php?post_type=nerfherder_post_type';
	?>
	 <script>location.href='<?php echo $url;?>';</script>
	<?php
}

function nerfherder_render_new_nerferhders(){
	$url = admin_url().'post-new.php?post_type=nerfherder_post_type';
	?>
	<script>location.href='<?php echo $url;?>';</script>
	<?php
}


/**
 * Hide default Custom Post Type Menu Item from Admin Menu
 */

add_action( 'admin_menu', 'nerfherder_remove_cpt_menu_items' );

function nerfherder_remove_cpt_menu_items() {
	remove_menu_page( 'edit.php?post_type=nerfherder_post_type' );
}


/**
 * Add/Remove appropriate CSS classes to Menu so Submenu displays open and the Menu link is styled appropriately.
 */
function nerfherder_correct_current_menu(){
	$screen = get_current_screen();
	if ( $screen->id == 'nerfherder_post_type' || $screen->id == 'edit-nerfherder_post_type' ) {
	?>
	<script type="text/javascript">
	jQuery(document).ready(function($) {
		$('#toplevel_page_nerfherder-plugin').addClass('wp-has-current-submenu wp-menu-open menu-top menu-top-first').removeClass('wp-not-current-submenu');
		$('#toplevel_page_nerfherder-plugin > a').addClass('wp-has-current-submenu').removeClass('wp-not-current-submenu');
	});
	</script>
	<?php
	}

	if ( $screen->id == 'nerfherder_post_type' ) {
	?>
	<script type="text/javascript">
	jQuery(document).ready(function($) {
		$('a[href$="nerfherder_plugin_add_post"]').parent().addClass('current');
		$('a[href$="nerfherder_plugin_add_post"]').addClass('current');
	});
	</script>
	<?php
	}

	if ( $screen->id == 'edit-nerfherder_post_type' ) {
	?>
	<script type="text/javascript">
	jQuery(document).ready(function($) {
		$('a[href$="nerfherder_plugin_show_posts"]').parent().addClass('current');
		$('a[href$="nerfherder_plugin_show_posts"]').addClass('current');
	});
	</script>
	<?php
	}
}

add_action('admin_head', 'nerfherder_correct_current_menu', 50);

Thank you for the hack. In WP 4.0 which I'm using, I can simply set 'show_in_menu' in custom post type args to $menu_slug that I set in add_menu_page() that I want to set the CPT as sub menu of and set the priority of a top-level menu function to 9 or lower.

Thanks for that snippet but to handle custom urls you don't have to use javascript redirection. You can go with something like that:

add_menu_page( __( 'Cposts', $this->plugin_name ), __( 'Cposts', $this->plugin_name ), 'manage_options', '/edit.php?post_type=cpost_post_type', '', 'dashicons-format-gallery', 6 );
add_submenu_page( '/edit.php?post_type=cpost_post_type', __( 'All Cposts', $this->plugin_name ), __( 'All Cposts', $this->plugin_name ), 'manage_options', '/edit.php?post_type=cpost_post_type', '');
add_submenu_page( '/edit.php?post_type=cpost_post_type', __( 'Add New Cpost', $this->plugin_name ), __( 'Add New Cpost', $this->plugin_name ), 'manage_options', '/post-new.php?post_type=cpost_post_type', '');

so you can declare fixed url as menu slug and pass null for function param. Current menu is handled natively in that case.