function use_cache ( $key = '' )
		{
			if (empty($_POST))
			{
				$this->cache_filename  =  $this->cache_dir . 'cache_' . md5($_SERVER['REQUEST_URI'] . serialize($key)) . '.ser';
				if (($_SERVER['HTTP_CACHE_CONTROL'] != 'no-cache')  &&  ($_SERVER['HTTP_PRAGMA'] != 'no-cache')  &&  @is_file($this->cache_filename))
				{
					if ((time() - filemtime($this->cache_filename)) < $this->cache_lifetime)
					{
						readfile($this->cache_filename);
						exit;
					}
				}
				ob_start( array( &$this, 'cache_callback' ) );
			}
		}