preventing double form submit

<?php
ob_start();
if (isset($_POST['send']) && $_POST['send']=='Send') {
  /* save_form_data_to_database(); if(some_error) {echo 'There was an error!';} else */
  { header('Location: '.$_SERVER['PHP_SELF'].'?status=ok'); exit; }
}
$str = ob_get_contents(); // get all printed text
ob_end_clean();
?><!DOCTYPE html>
<html>
<head>
<title>test for http://stackoverflow.com/questions/17028567/php-self-and-redirect-in-same-page</title>
<style type="text/css">
</style>
<script type="text/javascript">
</script>
</head>
<body>
  <form name="myform" action="" method="post">
    <input type="text" name="data1" value="aaa" />
    <input type="submit" name="send" value="Send" />
  </form>
<?php
  var_export($_POST);
  var_export($_GET);
  echo '<p> error message: '.$str.'</p>';
?>
</body>
</html>