<?php

// Setting a cookie when the user has successfully submitted an entry
// into our Guestbook.

setcookie( 'BGB_Data[ts]', $thetime, time()+60*60*24, '/gb/', '.example.net', 0 ); 

// 'BGB_Data[ts]' = the cookie name as an array; so you may add more data at a later stage
//                  if you want.
// $thetime = some data / value to create, verify or modify (depending how you want to
//            manage cookies)
// time()+60*60*24 = Cookie expires in 1 day.
// '/gb/' = the path, iow the cookie is available only within this folder and subfolders
//          therein.
// '.example.net' = so that the cookie is available regardless of whether the user uses
//                  http://www.example.net/gb/ or http://example.net/gb/ to get to your
//                  script.

?>

<?php

// Before we add this guestbook entry to our db, we
// verify that a cookie already exist (easy)
if( isset($_COOKIE['BGB_Data']['ts']) ):
  echo '<h1>Our records show that you already posted recently</h1>';
  exit;
endif;

// or verify that a cookie and the data we seek was already set (just a little less easy)
if( isset($_COOKIE['BGB_Data']['ts']) && $_COOKIE['BGB_Data']['ts'] >= time()-60*60*24 ):
  echo '<h1>Our records show that you already posted recently</h1>';
  exit;
endif;
?>

<?php
ob_start();

// the rest of your script below
?>