Possible Duplicate:
Invalid argument supplied for foreach()

Ben aşağıdaki kodu vardır:

<?
foreach($format as $form)
{
    echo $form;
    ?>
    <ul>
        <?
        $s = $database->onlineFormatUsers($form);
        while($row=mysql_fetch_assoc($s))
        {
            $username=$row['username'];
            $id=$row['id'];?>
            <li><a href="../userprofile.php?id=<?echo $id?>"><?echo "$username";?></a></li>
        <?
        }
        ?>
    </ul>
    <?
}
?>

<? 
//the active formats
$f = $database->activeFormats();
while($row=mysql_fetch_assoc($f))
{
    $format=$row['name'];
}
?>

It is saying its an invalid argument? Any reason why? Thanks