Using js load in same php file

 setInterval(function(){
         $.ajax({
               url :'user_online.php',

               success: function()
                   {
                       $('.chat-o').load('user_online.php');
                   }
            });
     },1000); 

Let's say I have this script. How do I make it work if user_online is the file I put this in (meaning, how can I make this script work internally-- within a php file called the same as the url)