var xmlHttp;

function checkavailability(){

	document.getElementById("caption_availability").innerHTML="Checking...";
	xmlHttp=GetXmlHttpObject();

	if (xmlHttp==null){
		alert ("Your browser does not support AJAX!");
		return;
	} 

	var url="ajaxcode/check_availability.php";
	var str = "" + document.getElementById("user_id").value;
	
	if(str == ""){
		document.getElementById("caption_availability").innerHTML = "enter a username";
		return;
	}
	
	url=url+"?id="+str;
	xmlHttp.onreadystatechange=stateChanged;
	xmlHttp.open("GET",url,true);
	xmlHttp.send(null);
} 

function stateChanged(){ 
	if (xmlHttp.readyState==4){ 
		document.getElementById("caption_availability").innerHTML=xmlHttp.responseText;
	}
} 

function GetXmlHttpObject(){
	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");
		}
	}
	return xmlHttp;
}