jQuery Ajax returns the whole page

I have a jquery-ajax function that sends data to a php script and the problem is with the return value, it returns the whole page instead of single value.

$("#ajaxBtn").click(function(){
  var inputText = $("#testText").val();
                    
  $.ajax({ type: "POST",
    url: "index.php",
    data: "testAjax="+inputText,
    dataType: "html",
    success: function(html){
      alert(html);
    }
  });
});         

That's how it works. You're requesting index.php via AJAX, so you'll get whatever the contents of index.php are.

If you want a particular value, make a new PHP page that outputs just that value, and request that URL's contents instead.