cookies are making me angry

[Anon]

alrighty, im trying to set this cookie:
$num = mysql_numrows($result);
if ($num != 0) {
setcookie(auth, ok, time()+604800, "/titanwarfare/");
$display_block = "
<p>
<h1><a href=\"addnews.php\">hi $username continue to admincenter</a></h1>
</P>
";

but i get this error:
Warning: Cannot add header information - headers already sent by (output started at /home/edgenetwork/www.bandwnation.com/titanwarfare/admin/loggedin.php:12) in /home/edgenetwork/www.bandwnation.com/titanwarfare/admin/loggedin.php on line 36
line 36 is setcookie
and line 12 is "<?" so im really confused about why I am getting this error...

[jjoske]

You have to set cookies before you send any headers to the browser. Have you already printed something before this part of the code?

[Anon]

yes i had html before it, but i fixed it, thanks