точно так

<?
$k_post=mysql_num_rows(mysql_query("SELECT DISTINCT `id_user` FROM `forum_t` "));
$k_page=k_page($k_post,$set['p_str']);
$page=page($k_page);
$start=$set['p_str']*$page-$set['p_str'];
$q=mysql_query("SELECT `id_user`, COUNT(*) AS `total` FROM `forum_t` GROUP BY `id_user` ORDER BY `total` DESC LIMIT $start, $set[p_str]");

if (mysqli_num_rows($q)>0)
{
$class='nav1';
while($data=mysqli_fetch_array($q,MYSQLI_ASSOC))
{
$ank=get_user($data['id_user']);
echo "<div class='$class'>";
echo $ank['nick']." <br />";
echo "Тем: $data[total] <br />";
echo "</div>";
$class=($class=='nav1'?'nav2':'nav1');
}


}else{ echo "<div class='err'>Нет данных</div>"; }

if ($k_page>1){str("?m=$mode&amp;",$k_page,$page);}
?>

Там исправь mysqli_num_rows
на mysql_num_rows

спасибо брать за помощ!