<?php  session_start();
if ($_GET['message'] == "success" && $_SESSION['revisit'] == "0") {
$_SESSION['revisit'] = "1";     ?>
<script type="text/javascript">window.alert('Message successfully posted.');</script>
<?php } ?>

if ($_GET['message'] == "success" && $_SESSION['revisit'] == "0") {

isset($_GET['message'])

if (isset($_GET['message']) && $_GET['message'] == "success" && isset($_SESSION['revisit']) && $_SESSION['revisit'] == "0") {

if (isset($_GET['message']) && $_GET['message'] == 'success' && ...) {
  ...
}

$message = isset($_GET['message']) ? $_GET['message'] : 'no_message';
if ($message == 'success' && ..) {...

if (isset($_GET['message']) && $_GET['message'] == "success" && $_SESSION['revisit'] == "0") {

if(isset($_GET['message']))