PHP Help, Plz. (Header set)?

While creating login page for my website, when I try to redirect my user to some page after successful login, I get this error:

Warning: Cannot modify header information - headers already sent by (output started at X:\somepath\common\db.php:20) in X:\common\member\login.php on line 64

Can someone help plz.

PHP Help, Plz. (Header set)?

You can't have the header() function do anything after you've had the page output anything, such as an echo(); or print();