singletonplugin.php

<?php
/*
Plugin Name: My Plugin
Version: 0.1
*/

class JWF_Plugin
{

	/**
	 * Holds the only instance of this plugin
	 *
	 * @static
	 * @var JWF_Plugin
	 * @access private
	 * @since 0.1
	 */
	private static $_instance = NULL;
	
	/**
	 * Plugin version
	 *
	 * @var string
	 * @access protected
	 * @since 0.1
	 */
	protected $version = '0.1';
	
	/**
	 * The absolute path of the plugin file
	 *
	 * @var string
	 * @access protected
	 * @since 0.1
	 */
	protected $plugin_file_path = '';
	
	/**
	 * The absolute path of the plugin directory
	 *
	 * @var string
	 * @access protected
	 * @since 0.1
	 */
	protected $plugin_dir_path = '';
	
	/**
	 * Constructor
	 *
	 * @since 0.1
	 */
	private function __construct()
	{
		// Actions
		add_action( 'plugins_loaded', array( &$this, 'finish_setup' ) );
		add_action( 'init', array( &$this, 'handle_content_types' ) );
		
		require_once $this->get_plugin_dir_path() . 'lib/fields.php';
		require_once $this->get_plugin_dir_path() . 'lib/widgets.php';
		
		if ( is_admin() ) {
			require_once $this->get_plugin_dir_path() . 'lib/admin.php';
		}
		
		// Activation hook
		register_activation_hook( $this->get_plugin_file_path(), array( &$this, 'plugin_activate' ) );
	}
	
	/**
	 * Get the instance of this class, insantiating it if it doesn't exist yet
	 *
	 * @since 0.1
	 *
	 * @return JWF_Plugin Class instance
	 */
	public static function get_instance()
	{
		if ( !is_object( self::$_instance ) ) {
			self::$_instance = new JWF_Plugin();
			
		}
		
		return self::$_instance;
	}
	
	/**
	 * Handle final aspects of plugin setup, such as adding action hooks
	 *
	 * @since 0.1
	 */
	public function finish_setup()
	{
		do_action( 'jwf/after_setup', &$this );
	}
	
	/**
	 * Handle inital installation and upgrading of the plugin
	 *
	 * @since 0.1
	 */
	public function plugin_activate()
	{
		$version = $this->get_version();
		$db_version = get_option( 'jwf_version' );
		
		$difference = version_compare( $db_version, $version );
		
		if ($difference != 0) {
			// Save new version
			update_option( 'jwf_version', $this->get_version() );
		}
	}
	
	/**
	 * Register and handle content types
	 */
	public function handle_content_types()
	{
		// Register post types and such
	}
	
	/**
	 * Get the main plugin file path
	 *
	 * @since 0.1
	 *
	 * @return string Absolute path to the main plugin file
	 */
	public function get_plugin_file_path()
	{
		if ( empty( $this->plugin_file_path ) ) {
			$this->plugin_file_path = realpath(__FILE__);
		}
		
		return $this->plugin_file_path;
	}
	
	/**
	 * Get the main plugin file path
	 *
	 * @since 0.1
	 *
	 * @return string Absolute path to the main plugin file
	 */
	public function get_plugin_dir_path()
	{
		if ( empty( $this->plugin_dir_path ) ) {
			$this->plugin_dir_path = plugin_dir_path( $this->get_plugin_file_path() );
		}
		
		return trailingslashit( $this->plugin_dir_path );
	}
	
	/**
	 * Get the plugin version
	 *
	 * @since 0.1
	 *
	 * @return string Plugin version
	 */
	public function get_version()
	{
		return $this->version;
	}

}

JWF_Plugin::get_instance();
?>