How about this ?

After vote I Found This warning on the top of page :

Warning: Cannot modify header information - headers already sent by (output started at /home/wanida/public_html/indexcoba.php:2) in /home/wanida/public_html/poll/poll_cookie.php on line 21

How to fix it ?

I don't use the poll myself but I'd say it uses cookies to prevent people cheating. Have you put any html before the poll because what this means is that you have already sent some information to the browser when it tries to send the cookies.

have a look thru the file you included the poll in get rid of any white spaces
like empty line breaks and spaces ' '.

Mainly before and after the cookie code and at the end of the file.

- Auron