commercial-client.php

<?php
/*
 * Plugin Name: Commercial Client
 * Plugin URI: http://pento.net/
 * Description: A sample client plugin for showing updates for non-WordPress.org plugins
 * Author: pento
 * Version: 0.1
 * Author URI: http://pento.net/
 * License: GPL2+
 */

// Don't allow the plugin to be loaded directly
if ( ! function_exists( 'add_action' ) ) {
	echo "Please enable this plugin from your wp-admin.";
	exit;
}

class CommercialClient {
	// This is the URL of the page we defined in CommercialServer.
	private $APIurl = 'http://localhost/api/';

	// This is the URL for your own changelog. You should make it look pretty.
	private $changelogURL = 'http://localhost/changelog/';

	// This should match the slug that CommercialServer::createAPI() sends
	private $slug = 'commercial-client';

	function init() {
		static $instance;

		if ( empty( $instance ) )
			$instance = new CommercialClient();

		return $instance;
	}

	function CommercialClient() {
		// WordPress doesn't let us hook directly into the update process.
		// The update info is storted in the 'update_plugins' transient, however,
		// so we can hook into that, instead.
		add_filter( 'pre_set_site_transient_update_plugins', array( $this, 'checkForUpdates' ), 10, 1 );

		// The changelog isn't available on WordPress.org, so we need to override the update information
		add_action( 'install_plugins_pre_plugin-information', array( $this, 'overrideUpdateInformation' ), 1 );
	}

	function checkForUpdates( $value ) {
		include ABSPATH . WPINC . '/version.php';

		$plugin_data = get_plugin_data( __FILE__ );

		// If your plugin has a key that you want to check on the server side,
		// you should include it in the 'body' element of this array.
		$options = array(
			'timeout'    => ( ( defined('DOING_CRON') && DOING_CRON ) ? 30 : 3 ),
			'user-agent' => 'WordPress/' . $wp_version . '; ' . get_bloginfo( 'url' ),
			'body'       => array(
							'version' => $plugin_data['Version'],
			)
		);

		$raw_response = wp_remote_post( $this->APIurl, $options );

		if ( is_wp_error( $raw_response ) || 200 != wp_remote_retrieve_response_code( $raw_response ) )
			return $value;

		$response = maybe_unserialize( wp_remote_retrieve_body( $raw_response ) );

		if ( empty( $response ) )
			return $value;

		// If you decide to have the server return different data (for example, for an invalid key),
		// you should process it here, then return $value, unmodified.

		$value->response[ plugin_basename( __FILE__ ) ] = $response;

		return $value;
	}

	function overrideUpdateInformation() {
		if ( wp_unslash( $_REQUEST['plugin'] ) !== $this->slug )
			return;

		wp_redirect( $this->changelogURL );
		exit;
	}
}

add_action( 'plugins_loaded', array( 'CommercialClient', 'init' ) );

commercial-server.php

<?php
/*
 * Plugin Name: Commercial Server
 * Plugin URI: http://pento.net/
 * Description: A sample server for showing updates for non-WordPress.org plugins
 * Author: pento
 * Version: 0.1
 * Author URI: http://pento.net/
 * License: GPL2+
 */

// Don't allow the plugin to be loaded directly
if ( ! function_exists( 'add_action' ) ) {
	echo "Please enable this plugin from your wp-admin.";
	exit;
}

class CommercialServer {
	// When the client site checks for an update, it will request this page.
	// We override it, for great justice.
	private $APIpageID = 26;

	private $currentVersion = '0.2';

	function init() {
		static $instance;

		if ( empty( $instance ) )
			$instance = new CommercialServer();

		return $instance;
	}

	function CommercialServer() {
		add_action( 'parse_query', array( $this, 'createAPI' ) );
	}

	function createAPI() {
		global $wp_query;

		if ( empty( $wp_query->queried_object ) )
			return;

		if ( ! is_a( $wp_query->queried_object, 'WP_Post' ) )
			return;

		if ( $this->APIpageID !== $wp_query->queried_object->ID )
			return;

		if ( empty( $_REQUEST['version'] ) )
			exit;

		// If there's no update, don't bother returning any info
		if ( version_compare( $this->currentVersion, $_REQUEST['version'], '<=' ) )
			exit;

		// If you want to have the client send a licence key, here would be a good place to check it.

		// So, here's a sample update. You probably want to fill this info from your version DB, or something.
		$response = new stdClass();
		$response->id = 0;
		$response->slug = 'commercial-client';
		$response->new_version = $this->currentVersion;
		$response->url = 'http://localhost/';
		$response->package = 'http://localhost/downloads/commercial-client-0.2.zip';

		echo serialize( $response );

		exit;
	}
}

add_action( 'plugins_loaded', array( 'CommercialServer', 'init' ) );