Código PHP:
<?php
if($_POST['yes'] == '1'){
$username= $_POST['username'];
$password= $_POST['password'];
$password2= $_POST['password2'];
$email= $_POST['email'];
$firstname= $_POST['firstname'];
$lastname= $_POST['lastname'];
$day = $_POST['day'];
$month = $_POST['month'];
$year = $_POST['year'];
$country = $_POST['country'];
//validar campos que esten llenos.
if ($username == "" && $password == "" && $email == "" && $firstname == "" && $lastname == "" )
{ //abro el if
?>
<SCRIPT LANGUAGE="javascript">
alert("Please fill all fields...");
history.go(-1);
</SCRIPT>
<?
}