process.php

<?php
   if ($_GET['key'] == $_SESSION['hash_key']) {
        //has key validation passed
   }
?>

registration.php

<?php
    //Generate hash
    $hash_key = md5($time);
    //Store hash in the SESSION 
    $_SESSION['hash_key'] = $hash_key;
?>
<form id="myform">
  <input type="text" name="my_name" />
  <input type="button" value="submit" onclick="frmSubmit()" />
  <script type="text/javascript>
       function frmSubmit() {
            document.forms["myform"].action="process.php?key=<?echo $hash_key;?>";
            document.forms["myform"].submit();
        }
  </script>
</form>