function createRequestObject(){
	var request_o;
	var browser = navigator.appName;
	if(browser == "Microsoft Internet Explorer"){
		request_o = new ActiveXObject("Microsoft.XMLHTTP");
	}else{
		request_o = new XMLHttpRequest();
	}
	return request_o;
}

var http = createRequestObject(); 

function checkUsername(){
	http.open('get', 'http://www.bppoddarhospital.net/_Admin/check_availability.php?id=' + document.frm_registration.user_name.value);
	http.onreadystatechange = displayMessage; 
	http.send(null);
}

function displayMessage(){
	if(http.readyState == 4){ 
		var response = http.responseText;
		alert(response);
	}
}