class PluginClass
{
    public static $instance = null;
    
    public static function init()
    {
        if ( null === self::$instance ) {
            self::$instance = new PluginClass();
            self::$instance->boot();
        }

        return self::$instance;
    }
    
    protected function __construct()
    {
        // Startup
    }
    
    protected function boot()
    {
        // Boot
    }
}

PluginClass::init();

class PluginClass
{
    protected static $instance = null;
    
    public function __construct($file)
    {
        if (static::$instance !== null) {
            throw new Exception;
        }

        static::$instance = $this;
    }

    public static function get()
    {
        return static::$instance;
    }
}

call_user_func(array(new PluginClass(__FILE__), 'boot'));