Cookie Won't Set.

I just made a login script and I made it so the user could check if they want to stay logged in forever. But when they check the box the cookie won't initiate. If they don't check the box, the session registers just fine.

<?php

if (isset($_POST['stay_logged'])){ 
      setcookie("sid", $row['userID'], time()+360000); 
} else { 
      $_SESSION['sid'] = $row['userID']; 
}?>

I made sure that $_POST['stay_logged'] was set, so thats not the problem.