Ajax

I find a lot of this AJAX stuff a bit of a hype.
Lots of people have been using similar things long
before it became "AJAX".  And it really isn't as
complicated as a lot of people make it out to be.
Here is a simple example from one of my apps.
First the Javascript:

 function createRequestObject() {

    var ro;

    var browser = navigator.appName;

    if(browser == "Microsoft Internet Explorer"){

        ro = new ActiveXObject("Microsoft.XMLHTTP");

    }else{

        ro = new XMLHttpRequest();

    }

    return ro;

}

 var http = createRequestObject();

 function sndReq(action) {

    http.open('get', 'rpc.php?action='+action);

    http.onreadystatechange = handleResponse;

    http.send(null);

}

 function handleResponse() {

    if(http.readyState == 4){

        var response = http.responseText;

        var update = new Array();

        if(response.indexOf('|' != -1)) {

            update = response.split('|');

    document.getElementById(update[0]).innerHTML = update[1];

        }

    }

}

This creates a request object along with a send request and handleresponse function.
So to actually use it, you could include
this js in your page.  Then to make one of these backend requests
you would tie it to something.  Like an onclick event or a straight
href like this:

   [foo]

 That means that when someone clicks on that link what actually
happens is that a backend request to rpc.php?action=foo will be
sent. In rpc.php you might have something like this:

   switch($_REQUEST['action']) {

    case 'foo':

      / do something /

      echo "foo|foo done";

      break;

    ...

  }

Now, look at handleResponse.  It parses the "foo|foo done" string
and splits it on the '|' and uses whatever is before the '|' as the
dom element id in your page and the part after as the new innerHTML
of that element.  That means if you have a div tag like this in
your page:

Once you click on that link, that will dynamically be changed to:   

  foo done
That's all there is to it.  Everything else is just building on
top of this.  Replacing my simple response "id|text" syntax with
a richer XML format and makine the request much more complicated
as well.Before you blindly install large "AJAX" libraries, have a
go at rolling your own functionality so you know exactly how it
works and you only make it as complicated as you need.
Often you don't need much more than what I have shown here.

Expanding this approach a bit to send multiple parameters in the

request, for example, would be really simple.  Something like:

   function sndReqArg(action,arg) {

    http.open('get', 'rpc.php?action='+action+'&arg='+arg);

    http.onreadystatechange = handleResponse;

    http.send(null);

  }

 And your handleResponse can easily be expanded to do much more

interesting things than just replacing the contents of a div.