PHP/AJAX长轮询:返回可用的新内容

I am creating a PHP/AJAX chat client. The application makes an AJAX request to a PHP script which gets the contents of an XML file, and responds with the contents. The client then parses the XML and displays the chats. The XML file contains all the chats, but to save server resources, I would only like the PHP script to return new chats, instead of all the chats. Here is my XML file:I am creating a PHP/AJAX chat client. The appli