Could anyone show me how would I make this cookie last 30 days?

Hi all, I have this code in my page to set a cookie. Thing is it is only a session cookie :-( Would appreciate it if someone show me how I make it last 30 days please?

<?php
if (isset($_GET['kw']))
  {
  setcookie('keyword', $_GET['kw']);
  $kw = $_GET['kw'];
  }
if (isset($_COOKIE['keyword']))
  {
  $kw = $_COOKIE['keyword'];
  }
?>

Code (markup):

Thanks

Mike