<a href='javascript:;' onClick='doAjaxRequest("ajaxexample.php?action=printSimpleMessage", "responseDiv");'>
	Click here to receive a message from the server
</a>

<div id='responseDiv'>The server message will go here</div>

function doAjaxRequest(actionUrl, elemId) {

// MAKE NEW XMLHTTP OBJECT
	var xmlHttp=null
	try { // Firefox, Opera 8.0+, Safari
		xmlHttp=new XMLHttpRequest()
	}
	catch (e) { // Internet Explorer
		try {
			xmlHttp=new ActiveXObject("Msxml2.XMLHTTP")
		}
		catch (e) {
			xmlHttp=new ActiveXObject("Microsoft.XMLHTTP")
		}
	}

	if (xmlHttp==null) {
		alert ("Browser does not support HTTP Request")
		return
	}

// WHAT TO DO WITH THE RESPONSE
	xmlHttp.onreadystatechange=function() {
		if (xmlHttp.readyState==1 || xmlHttp.readyState=="loading") {
			document.getElementById(elemId).innerHTML="processing, please wait..."
		}
		else if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete") {
			document.getElementById(elemId).innerHTML=xmlHttp.responseText
		}
	}

// MAKE THE REQUEST
	xmlHttp.open("GET",actionUrl,true)
	xmlHttp.send(null)
}

<?
if(isset($_GET['action'])) {
	$gaction = $_GET['action'];
	if($action == 'printSimpleMessage') echo "Here is a message from the server";
	
	exit;
}
?>