Custom admin boxes example

<?php
/*
Plugin Name: Test p2p
*/
add_action('init', 'project_testp2p_init');
add_action( 'add_meta_boxes', array('project_testp2p_editor', 'register_p2p_meta_box'));
add_action( 'wp_loaded', 'project_testp2p_wp_loaded' );
function project_testp2p_init()
{
    $labels = array(
        'name' => _x('Sub Posts', 'post type general name'),
        'singular_name' => _x('Sub Post', 'post type singular name'),
        'add_new' => _x('Add New Sub Post', 'Event'),
        'add_new_item' => __('Add New Sub Post'),
        'edit_item' => __('Edit Sub Post'),
        'new_item' => __('New Sub Post'),
        'view_item' => __('View Sub Post'),
        'search_items' => __('Search Sub Posts'),
        'not_found' =>  __('No Sub Posts found'),
        'not_found_in_trash' => __('No Sub Posts found in Trash'),
        'parent_item_colon' => '',
    );

    $supports = array('editor', 'revisions', 'comments', 'author', 'custom-fields', 'title');

    register_post_type( 'subpost',
                        array('labels' => $labels,
                             'public' => true,
                             'show_in_menu' => true,
                             'supports' => $supports,
                             'hierarchical' => true
                        ));
}
function project_testp2p_wp_loaded() {
    // Make sure the Posts 2 Posts plugin is active.
    if ( !function_exists( 'p2p_register_connection_type' ) )
        return;

    p2p_register_connection_type(
        array(
            'name' => 'subpost_to_post',
            'from' => 'subpost',
            'to' => 'post',
            'cardinality' => 'many-to-one',
            'admin_box' => 'any',
            'admin_column' => 'any',
        )
    );
}
class project_testp2p_editor {
    static function register_p2p_meta_box() {
        add_meta_box( 'project-ace-program-box', 'Link SubPost', array('project_testp2p_editor', 'render_post_meta_box'), 'post' );
        add_meta_box( 'project-ace-program-tab-box', 'Link Post', array('project_testp2p_editor', 'render_program_subpost_meta_box'), 'subpost' );
    }

    static function render_post_meta_box( $post ) {
        p2p_list_posts( p2p_type('subpost_to_post')->get_connected( $post->ID ));
    }

    static function render_program_subpost_meta_box( $post ) {
        p2p_list_posts( p2p_type('subpost_to_post')->get_connected( $post->ID ) );
    }
}
?>