Updated WP to 2.8 , then I soloved a permission problem of phpbb3. The owner of some files created by PHP is nobody , and the Public permissions of them is 0 ,that means other users can not read them at least, so I  could not backup files via ftpclient and I could not chmod via ftpclient  obviously.

I wrote some codes,

<?php
$sign=chmod(‘filename’,0644);
echo $sign;
?>

Then uploaded it to the directories via ftpclient and  ran them via web browser, all things became OK.