I just learned new errors about the Undefined index but at this time i just can't figure out how to solve this, i got error said

Notice: Undefined index: userid in C:\VertrigoServ\xxx \login.php on line 104

which is the codes showed

$nb_new_pm = mysql_fetch_array(mysql_query('select count(*) as nb_new_pm from pm where ((user1="'.$_SESSION['userid'].'" and user1read="no") or (user2="'.$_SESSION['userid'].'" and user2read="no")) and id2="1"'));
$nb_new_pm = $nb_new_pm['nb_new_pm'];
and other one in same error online 112 showed

<a href="list_pm.php">Your messages(<?php echo $nb_new_pm; ?>)</a> - <a href="profile.php?id=<?php echo $_SESSION['userid']; ?>"><?php echo htmlentities($_SESSION['username'], ENT_QUOTES, 'UTF-8'); ?></a> (<a href="login.php">Logout</a>)
after I put ini_set('display_errors','Off'); to remove error notice but is did not remove.

Does anyone know what the problem relate to undefined index...

please help thanks.