javascript required input change input border

im trying to change the border if the user has leave the input empty after posting

well, i have tried:

<script>
   function required (id) {
       document.getElementById(id).style.border = "#FF0000 5px inset";
   }
</script>
<form method="post">
<input type='text' id='req' name='t'>
</form>
<?
if ($_POST)
   if (isset($_POST['t']))
        echo "u've done";
   else
        echo "<script>required('req');</script>";
?>