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<html>
<head>

<script type="text/javascript">
   function validateThis() {
       var username = document.getElementById('username');
       var password = document.getElementById('userpass');
       if ((username.value =="") || (password.value == "")) {
        alert("Please enter your details");
        return false;
      }

   else {
      return true;
     }
}
</script>
</head>

<body>
    <form id="myForm" name="myForm" action="loginValidate.php"
    method="get" onSubmit="validateThis()">

    <input type="text" name="username"/>
    <input type="password" name="userpass"/>
    <input type="submit" value="Login"/>

    </form>

</body>
</html>

<?php

$username = $_GET ['username'];
echo "Hello".$username;

?>

<html>
<head>
<script type="text/javascript">
function validateThis(){
  if(document.forms['myForm'].username.value == "")
  {
    alert("empty");
    return false;
  }
    return true;
}
</script>
</head>
<body>
    <form id="myForm" name="myForm" action=""
    method="get" onSubmit="validateThis()">
    <input type="text" name="username"/>
    <input type="submit" value="Login"/>
    </form>
</body>
</html>
<?php
echo !empty($_GET['username']) ? "Hello ". $_GET['username'] : "";
?>