when I use extern.php like this
"extern.php?action=new&show=10"
there is some problem with multibyte charset
so I change
$subject_truncated = pun_htmlspecialchars(trim(substr($cur_topic['subject'], 0, ($max_subject_length-5)))).' …';
to
$subject_truncated = pun_htmlspecialchars(trim(mb_substr($cur_topic['subject'], 0, ($max_subject_length-5)))).' …';
and problem was solved.