How to fix "Headers already sent" error in PHP

When running my script, I am getting several errors like this: > Warning: Cannot modify header information - headers already sent by (**output started at /some/file.php:12**) in **/some/file.php** on **line 23** The lines mentioned in the error messages contain [`header()`][1] and [`setcookie()`][2] calls. What could be the reason for this? And how to fix it? [1]: http://php.net/header [2]: http://php.net/setcookie