错误 - php提示「headers already sent by」是怎么回事?

错误提示:
SAE_Warning: Cannot modify header information - headers already sent by (output started at /data1/www/htdocs/345/leank/1/bbs/login.php:15) in bbs/login.php on line 9

SAE_Warning: Cannot modify header information - headers already sent by (output started at /data1/www/htdocs/345/leank/1/bbs/login.php:15) in bbs/login.php on line 10

SAE_Warning: Cannot modify header information - headers already sent by (output started at /data1/www/htdocs/345/leank/1/bbs/login.php:15) in bbs/login.php on line 28

SAE_Warning: Cannot modify header information - headers already sent by (output started at /data1/www/htdocs/345/leank/1/bbs/login.php:15) in bbs/login.php on line 29

大致的代码:

  ";
            }
            else
            {
                echo "登录失败!";
                echo "请检查您的用户名或密码是否都填写正确";
            }
        }
        else
        {
            echo "登录失败!";
            echo "请检查您的用户名或密码是否为空";
        }
    }
  ?>

处错误的大致是这个位置:
setcookie('username','$username',time()+31536000,'/');
setcookie('logset','ok',time()+31536000,'/');