upload file permission problem

Hi all,
  I just upgraded my server into PHP 5. I run the same script which is
working in 4.2.2.
But In PHP 5 has followings msg:
Warning: move_uploaded_file(upload_files/Gnatt.htm)
[function.move-uploaded-file]: failed to open stream: Permission
denied in /var/www/html/simple_upload.php on line 60

Warning: move_uploaded_file() [function.move-uploaded-file]: Unable to
move '/tmp/phpZ29DAN' to 'upload_files/Gnatt.htm' in
/var/www/html/simple_upload.php on line 60

I am 100% sure , I have changed the upload_files dir into 0777 mod.

Any one can help?

yours,
Michael Leung