Update/Upgrade class for WordPress plugins in repos outside wp.org (Original Class: March 8, 2011 by Ronald Huereca)

upgdate.php

<?php

//Plugin Upgrade Class
if ( !class_exists( 'PluginUpgrade' ) ) 
{

class PluginUpgrade 
{
	private $plugin_url			= false;
	private $remote_url			= false;
	private $version			= false;
	private $plugin_slug		= false;
	private $plugin_path		= false;
	private $time_upgrade_check	= false;
	private $plugins			= '';

	public function init() 
	{
		if ( !is_admin() )
			return;

		// Set up update checking and hook in the filter for automatic updates
		// Do upgrade stuff
		if ( current_user_can( 'manage_options' ) ) 
		{
			$this->check_periodic_updates();
			if ( isset( $this->plugins[$this->plugin_slug]->new_version ) ) 
			{
				if( !version_compare( $this->version, $this->plugins[$this->plugin_slug]->new_version, '>=' ) ) 
				{
					add_filter( 'site_transient_update_plugins', array( &$this, 'update_plugins_filter' ), 1000 );
				}
			}
		}
	}

	function __construct( $args = array() ) 
	{
		// Load defaults
		extract( wp_parse_args( 
			$args, 
			array(
				 'remote_url'	=> false
				,'version'		=> false
				,'plugin_slug'	=> false
				,'plugin_path'	=> false
				,'plugin_url'	=> false
				,'time'			=> 43200
			) 
		) );

		$this->plugin_url		= $plugin_url;
		$this->remote_url		= $remote_url;
		$this->version			= $version;
		$this->plugin_slug		= $plugin_slug;
		$this->plugin_path		= $plugin_path;

		$this->time_upgrade_check = apply_filters( 'pu_time_'.$plugin_slug, $time );

		// Get plugins for upgrading
		$this->plugins = $this->get_plugin_options();

		add_action( 'admin_init', array( &$this, 'init' ), 1 );
		add_action( 'after_plugin_row_'.$plugin_path, array( &$this, 'plugin_row' ) );

	}

	/**
	 * Update Check
	 * Performs a periodic upgrade check to see if the plugin needs to be upgraded or not
	 */
	private function check_periodic_updates() 
	{
		if ( isset( $this->plugins[ $this->plugin_slug ]->last_update ) )
		{
			$last_update = $this->plugins[ $this->plugin_slug ]->last_update;
		}
		else 
		{
			$last_update = false;
		}

		if ( !$last_update ) 
		{ 
			$last_update = $this->check_for_updates(); 
		}

		if( ( time() - $last_update ) > $this->time_upgrade_check )
		{
				$this->check_for_updates();
		}
	}

	/**
	 * 
	 * Enter description here ...
	 */
	public function get_remote_version() 
	{
		if ( isset( $this->plugins[ $this->plugin_slug ]->new_version ) ) 
		{
			return $this->plugins[ $this->plugin_slug ]->new_version;
		}
		
		return false;
	}

	/**
	 * Options - get
	 * Enter description here ...
	 */
	private function get_plugin_options() 
	{
		//Get plugin options
		if ( is_multisite() ) 
		{
			$options = get_site_option( 'pu_plugins' );
		} 
		else 
		{
			$options = get_option( 'pu_plugins' );
		}

		if ( !$options ) 
		{
			$options = array();
		}

		return $options;
	}

	/**
	 * Options - save
	 * Enter description here ...
	 */
	private function save_plugin_options() 
	{
		//Get plugin options
		if ( is_multisite() ) 
		{
			update_site_option( 'pu_plugins', $this->plugins );
		} 
		else 
		{
			$options = update_option( 'pu_plugins', $this->plugins );
		}
	}

	/**
	 * Check for update
	 * Enter description here ...
	 * 
	 * @param unknown_type $manual
	 */
	public function check_for_updates( $manual = false ) 
	{
		if ( !is_array( $this->plugins ) ) 
			return false;

		//Check to see that plugin options exist
		if ( !isset( $this->plugins[$this->plugin_slug] ) ) 
		{
			$plugin_options = new stdClass;
			$plugin_options->url			= $this->plugin_url;
			$plugin_options->slug			= $this->plugin_slug;
			$plugin_options->package		= '';
			$plugin_options->new_version	= $this->version;
			$plugin_options->last_update	= time();
			$plugin_options->id				= "0";

			$this->plugins[$this->plugin_slug] = $plugin_options;

			$this->save_plugin_options();
		}

		$current_plugin = $this->plugins[$this->plugin_slug];
		if( ( time() - $current_plugin->last_update ) > $this->time_upgrade_check || $manual ) 
		{
			//Check for updates
			$version_info = $this->perform_remote_request( 'get_version' );
			if ( is_wp_error( $version_info ) ) 
				return false;

			//$version_info should be an array with keys ['version'] and ['download_url']
			if ( isset( $version_info->version ) && isset( $version_info->download_url ) ) 
			{
				$current_plugin->new_version			= $version_info->version;
				$current_plugin->package				= $version_info->download_url;
				$this->plugins[ $this->plugin_slug ]	= $current_plugin;

				$this->save_plugin_options();
			}
		}

		return $this->plugins[$this->plugin_slug];
	}

	/**
	 * Action hook
	 * Enter description here ...
	 * 
	 * @param unknown_type $plugin_name
	 * @return void
	 */
	public function plugin_row( $plugin_name ) 
	{
		do_action( 'pu_plugin_row_'.$this->plugin_slug, $plugin_name );
	}

	/**
	 * Perform request
	 * Enter description here ...
	 * 
	 * @param unknown_type $action
	 * @param unknown_type $body
	 * @param unknown_type $headers
	 * @param unknown_type $return_format
	 * 
	 * @return boolean|string|Ambigous <mixed, boolean, NULL, number, unknown, multitype:, stdClass, string>
	 */
	public function perform_remote_request( $action, $body = array(), $headers = array(), $return_format = 'json' ) 
	{
		$body = wp_parse_args( 
			 $body
			,array(
				 'action'		=> $action
				,'wp-version'	=> get_bloginfo( 'version' )
				,'referer'		=> site_url()
			) 
		);
		$body = http_build_query( apply_filters( 'pu_remote_body_'.$this->plugin_slug, $body ) );

		$headers = wp_parse_args( $headers, array(
			'Content-Type'		=> 'application/x-www-form-urlencoded',
			'Content-Length'	=> strlen( $body )
		) );
		$headers = apply_filters( 'pu_remote_headers_'.$this->plugin_slug, $headers );

		$post = array( 
			 'headers'	=> $headers
			,'body'		=> $body 
		);

		//Retrieve response
		$response = wp_remote_post( esc_url( $this->remote_url ), $post );
		$response_code = wp_remote_retrieve_response_code( $response );
		$response_body = wp_remote_retrieve_body( $response );

		if ( $response_code != 200 || is_wp_error( $response_body ) ) 
		{
			return false;
		}

		if ( $return_format != 'json' ) 
		{
			return $response_body;
		} 
		else 
		{
			return json_decode( $response_body );
		}

		return false;
	}

	/**
	 * Update Filter
	 * Return an updated version to WordPress when it runs its update checker
	 * 
	 * @param unknown_type $value
	 * @return unknown_type $value
	 */
	public function update_plugins_filter( $value ) 
	{
		if ( isset( $this->plugins[ $this->plugin_slug ] ) && $this->plugin_path ) 
		{
			$value->response[ $this->plugin_path ] = $this->plugins[ $this->plugin_slug ];
		}
	
		return $value;
	}

} // END Class PluginUpgrade

} // endif;
?>

FYI - also recommended: https://github.com/YahnisElsts/plugin-update-checker together with https://github.com/YahnisElsts/wp-update-server - using these both 2 components for my self hosted plugins