Alternative PHP Cache is a PECL extension, which can be utilized for many purposes. The best thing is that it can check if a file has been modified on every request. Rasmus Lerdorf (you should know the name if you are a PHP guy) who was also a pioneer in the extension development, came up with a classic example.
The example is available in http://progphp.com/progress.php and you can view the source code here. This code example uses YUI code library for presentation layer and JSON for data transfer. apc_fetch function is being used to get the status from the cache. To use it fully all you need to do is to install PECL and this extension in your system. The rest is pure magic.
