Código PHP:
$sql2="SELECT * FROM `foro_cat` WHERE `admin` < ".$admin_user_level."+1";
$res2=mysql_query($sql2) or die(mysql_error());
while($row=mysql_fetch_assoc($res2)){
$sql3="SELECT * FROM `foro_sub_cat` WHERE `cid`='".$row['id']."'";
$res3=mysql_query(sql3) or die (mysql_error());
echo "<option value=\"0\">".$row['name']."</option>\n";
while($row2 = mysql_fetch_assoc($res3)){
$selected=($row2['id']==$id) ? " SELECTED" : "";
echo "<option value=\"".$row2['id']."\"".$selected."> ".$row2['name']."</option>\n";
}
}