[WordPress] Add a custom post type menu as a child of an existing custom post type menu.

add-custom-post-type-menu.php

<?php

// Define the 'Portfolio' post type. This is used to represent galleries
// of photos. This will be our top-level custom post type menu
$args = array(
  'labels'	=>	array(
            'all_items'           => 	'Gallery',
						'menu_name'	          =>	'Portfolio',
						'singular_name'       =>	'Gallery',
					 	'edit_item'           =>	'Edit Gallery',
					 	'new_item'            =>	'New Gallery',
					 	'view_item'           =>	'View Gallery',
					 	'items_archive'       =>	'Gallery Archive',
					 	'search_items'        =>	'Search Portfolio',
					 	'not_found'	          =>	'No galleries found',
					 	'not_found_in_trash'  =>	'No galleries found in trash'	
					),
	'supports'		=>	array( 'title', 'editor', 'author', 'revisions' ),				
	'menu_position'	=>	5,
	'public'		=>	true
);
register_post_type( 'portfolio', $args );

// Next, we'll define a second custom post type called 'Locations' where we could
// potentially display a list of locations that are used as part of our portfolio.
// This custom post type will be added as a submenu to the 'Portfolio' menu
$args = array(
  'labels'	=>	array(
						'all_items' => 	'Locations',
						'menu_name'	=>	'Locations',
						'singular_name' =>	'Location',
					 	'edit_item' =>	'Edit Location',
					 	'new_item'  =>	'New Location',
					 	'view_item' =>	'View Location',
					 	'items_archive' =>	'Location Archive',
					 	'search_items'  =>	'Search Locations',
					 	'not_found'	    =>	'No locations found.',
					 	'not_found_in_trash'  => 'No locations found in trash.'
					),
	'supports'      =>	array( 'title', 'editor', 'revisions' ),				
	'show_in_menu'  =>	'edit.php?post_type=portfolio', // This is where we tell WordPress to add 'Locations' as a submenu
	'public'		    =>	true
);
register_post_type( 'location', $args );