premium-plugin-class.php

<?php
class Premium_Plugin {



 	/**
	 * Plugin API Url
	 *
	 * @access protected
	 * @var string
	 **/
	protected $plugin_api_url = 'http://domain.tld';



	/**
	 * Plugin file
	 *
	 * @access protected
	 * @var string
	 **/
	protected $plugin_file;



	/**
	 * Plugin Slug
	 *
	 * @access protected
	 * @var string
	 **/
	protected $plugin_slug = 'premium-plugin';



	/**
	 * Plugin version
	 *
	 * @var string
	 **/
	protected $plugin_version = '1.0';



	/**
	 * Constructor
	 *
	 * @return void
	 * @author Ralf Hortt
	 **/
	public function __construct()
	{
		$this->plugin_file = plugin_basename( __FILE__ );

		add_filter( 'pre_set_site_transient_update_plugins', array( $this, 'api_has_update' ) );
		add_filter( 'plugins_api_result', array( $this, 'api_plugin_information' ), 10, 3 );
	}



	/**
	 * Call plugin API if an update exists
	 *
	 * @access public
	 * @param obj $transient Transient object
	 * @uses get_option
	 * @uses Contentboxes_PRO::api_request
	 * @return void
	 * @author Ralf Hortt
	 **/
	public function api_has_update( $transient )
	{
		// Check if the transient contains the 'checked' information
		// If no, just return its value without hacking it
		if ( empty( $transient->checked ) )
			return $transient;

		$options = get_option( 'plugin-settings-api' );
		if ( empty($options['license']) || '' == $options['license'] )
			return $transient;

		// The transient contains the 'checked' information
		// Now append to it information form your own API

		// POST data to send to your API
		$args = array(
			'action' => 'update-check',
			'plugin_name' => $this->plugin_slug,
			'version' => $this->plugin_version,
			'license' => $options['license'],
			'url' => get_bloginfo( 'wpurl' )
		);

		// Send request checking for an update
		$response = $this->api_request( $args );

		// If response is FALSE, don't alter the transient
		if ( FALSE !== $response ) :
			$transient->response[$this->plugin_file] = $response;
		endif;

		return $transient;
	}



	/**
	 * Return plugin details from alternate repository
	 *
	 * @access public
	 * @param $res
	 * @param $action
	 * @param $args
	 * @return void
	 * @author Ralf Hortt
	 **/
	public function api_plugin_information( $res, $action, $args )
	{
		if ( $this->plugin_slug != $args->slug )
			return $res;

		$options = get_option( 'plugin-settings-api' );

		// POST data to send to API
		$args = array(
			'action' => 'plugin-information',
			'plugin_name' => $this->plugin_slug,
			'version' => $this->plugin_version,
			'license' => $options['license'],
			'url' => get_bloginfo('wpurl')
		);

		// Send request for detailed information
		$response = $this->api_request( $args );

		return $response;
	}



	/**
	 * Send an update request
	 *
	 * @access public
	 * @param array $args Arguments
	 * @return void
	 * @author Ralf Hortt
	 **/
	public function api_request( $args )
	{
		// Send request
		$request = wp_remote_post( $this->plugin_api_url, array( 'body' => $args ) );

		// Make sure the request was successful
		if( is_wp_error( $request ) or wp_remote_retrieve_response_code( $request ) != 200 ) :
			// Request failed
			return FALSE;
		endif;

		// Read server response, which should be an object
		$response = unserialize( wp_remote_retrieve_body( $request ) );
		if ( is_object( $response ) ) :
			return $response;
		else :
			// Unexpected response
			return FALSE;
		endif;
	}



}

Found this here: http://wordpress.stackexchange.com/questions/113740/my-own-svn-for-a-plugin-theme