php设置cooke报错

Warning : Cannot modify header information - headers already sent by (output started at F:\php_work\bbs\header.php:6) in F:\php_work\bbs\test.php on line 3 （Cannot modify header information - headers already sent by）不能修改标题信息-标题

Warning: Cannot modify header information - headers already sent by (output started at F:\php_work\bbs\header.php:6) in F:\php_work\bbs\test.php on line 3

 

（Cannot modify header information - headers already sent by）不能修改标题信息-标题已发出　？？？？这什么意思