php - I can't setcookie "headers already sent"

Possible Duplicate:
“Warning: Cannot modify header information - headers already sent by” error
setcookie, Cannot modify header information - headers already sent

I have this code to set cookie :

if (!isset($_COOKIE["CorkIU"])) {

setcookie("CorkIU", 2dd2ee3aUgsvoRye, time()+60*60*24*365, "/");
}

I have added this code in layout.php file

The first lines from layout.php file is

    include ("inc/css.php");

if (!isset($_COOKIE["CorkIU"])) {
setcookie("CorkIU", 2dd2ee3aUgsvoRye, time()+60*60*24*365, "/");
}

?>

and here is the error :

Warning: Cannot modify header information - headers already sent by (output started at /home/ex/public_html/template/layout.php:4) in /home/ex/public_html/template/layout.php on line 8