A very simple booking management add-on for Event Organiser

<?php
/*
Plugin Name: My Event Organiser Booking Manager
Plugin URI: http://www.HarrisWebSolutions.co.uk/event-organiser
Description:
         A class that can run alongside the Event Organiser WordPress plugin.
         Adds a very simple booking management functionality to the plug-in,
         and can be used as a base for something more complicated. The 'add-on'
         adds a button to the bottom of single event pages. Clicking this
         'registers' the current user for that event by adding their ID as
         post meta. If the current user is registered a message is displayed
         instead informing them. Bookings for an event can be manually
         configured by editing the appropriate key-values in the custom fields
         metabox. Currently does not necessarily need Event Organiser (in
         particular) to run.
Author: Stephen Harris
Version: 1
*/
/*  Copyright 2011 Stephen Harris (stephen@harriswebsolutions.co.uk)
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    GNU General Public License for more details.
*/

class My_eo_bookings {
        //Name of the action used for nonces & action checks - should be unique to avoid classes with WP/Plugins
    static $action = 'my_eo_booking_add';
        //The meta key used to store users' ID. Again should be unique.
    static $key = 'my_eo_registered_users';

    static function load() {
        add_filter( 'init', array( __CLASS__, 'init' ));
    }

    static function init(){
        if(is_user_logged_in()){
            //Recommended to run hooked functions 'late', e.g. priority of 20 or more
            if(isset($_POST['action']) && $_POST['action']==self::$action)
                add_filter( 'init', array( __CLASS__, 'booking_insert' )  ,20);

            add_filter( 'the_content', array( __CLASS__, 'booking_markup' )  ,20);
        }
    }


    static function booking_markup($content ) {

        if(!is_singular('event'))
            return $content;

        //The booking mark-up html, in this example just a 'sign me up' button if the user is logged in
        $post_id = get_the_ID();
        $user = wp_get_current_user();
        $user_id = $user->ID;
        $nonce = self::$action.'_'.$post_id.'_'.$user_id;

        if(self::is_booked($user_id,$post_id)){
            $html = "<p>You registered for this event </p>";

        }else{
            $html = "<p><form method='POST'>";
            $html .= wp_nonce_field( $nonce,'_wpnonce',false,false);
            $html .= "<input type='hidden' name='".self::$action."' value='".$post_id."' />";
            $html .= "<input type='hidden' name='action' value='".self::$action."' />";
            $html .= "<input type='submit' value='Sign me up!' />";
            $html .= "</form></p>";
        }

        $content .= $html;

            return $content;
    }


    static function booking_insert() {
        $post_id = (isset($_POST[self::$action]) ? intval($_POST[self::$action]) : 0);
        $user = wp_get_current_user();
        $user_id = $user->ID;
        $nonce = self::$action.'_'.$post_id.'_'.$user_id;

        //booking_insert should only run for events, but just in case...
        if(get_post_type( $post_id)!='event')
            return;

        //If empty data / non-numeric data sent abort.
        if(empty($user_id) || empty($post_id))
            return;

        //Check nonce
        if (! wp_verify_nonce($_POST['_wpnonce'],$nonce) )
            die('Security check');

        //Nonces passed, perform any further checks if necessary then add user to post meta
        add_post_meta($post_id, self::$key, $user_id);
    }

    static function is_booked($user_id,$post_id){
        $user_id = (int) $user_id;
        $post_id = (int) $post_id;

        if(empty($user_id)||empty($post_id))
            return false;

        if(get_post_type( $post_id)!='event')
            return false;

        $registered = get_post_meta($post_id, self::$key);

        return in_array($user_id, $registered);
    }
}
My_eo_bookings::load();
?>