rpc.js
function createRequestObject() {
  var ro;
  ro = window.XMLHttpRequest 
       ? new XMLHttpRequest() : 
         new ActiveXObject("Microsoft.XMLHTTP");
  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 handle response 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:

 <a href="javascript:sndReq('foo')">[foo]</a>
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.

rpc.php
<?php
  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:

<div id="foo">
</div>
Once you click on that link, that will dynamically be changed to:

<div id="foo">
  foo done
</div>
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);
}