Jquery - 如何使ifModified：true在.ajax请求中工作？

I have a simple script where an Ajax request is made every 2 seconds. If the data fetched from a php page called "ext" is different from the last data acquired, the code runs a function: in order to do this I use the ifModified:true setting on the ajax request, but it doesn't work.I have a simple script where an Ajax request is