I have a one-page URL shortening website. When the shortened URL is clicked how can I display a page with an ad on for about 3 seconds?

<?php
$baseurl = 'http://zumco.co.uk/'; //address to use as the first part of the short URLs (note trailing slash)
$sql_host = 'localhost'; //host of your mysql database (usually localhost)
$sql_user = 'moseleya_moseley'; //mysql user with access to your database
$sql_pass = 'pass'; //password for this user
$sql_db = 'moseleya_turl'; //the name of your database
 
$db = mysql_connect($sql_host, $sql_user, $sql_pass);
mysql_select_db($sql_db, $db);
 
if(isset($_GET['k'])) {
   $k = mysql_real_escape_string($_GET['k'], $db);
   if($result = mysql_query("SELECT `url` FROM `turl` WHERE `key` = '" . $k . "'", $db)) {
      if(mysql_num_rows($result) > 0) {
         $row = mysql_fetch_row($result);
         header('HTTP/1.1 301 Moved Permanently');
         header('Location: ' . $row[0]);
         exit;
      }
   }
}
 
if(isset($_POST['submit'], $_POST['url'])) {
   if(filter_var($_POST['url'], FILTER_VALIDATE_URL, FILTER_FLAG_HOST_REQUIRED) !== false) {
      $url = mysql_real_escape_string($_POST['url'], $db);
      if($result = mysql_query("SELECT `key` FROM `turl` WHERE `url` = '" . $url . "'", $db)) {
         if(mysql_num_rows($result) > 0) {
            $row = mysql_fetch_row($result);
            $key = $row[0];
         }
      }
      if(!isset($key) && mysql_query("INSERT INTO `turl` (`url`) VALUES ('" . $url . "')", $db)) {
         $id = mysql_insert_id($db);
         $key = base_convert($id, 10, 36);
         mysql_query("UPDATE `turl` SET `key` = '" . $key . "' WHERE `id` = '" . $id . "'");
      }
      echo '<a href="' . $baseurl . $key . '" target="_blank">' . $baseurl . $key . '</a>';
   }
}
else {
?>
<form method="post" action="index.php">
<label>URL: <input type="text" name="url" id="url" /></label>
<br />
<input type="submit" name="submit" id="submit" value="Shorten" />
</form>
<?php
}
?>

You use an interstitial page with a javascript timer that calls the redirect URI on timeout.

Dont even need to get that convoluted

Use a <meta refresh

<meta http-equiv="refresh" content="5;url=newpage.php">