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:

<?php
/*
* 未完成内容 : 添加判断的url中log?= "" 的if语句,来判断用户是登陆还是退出
*添加一个判断是否已经登陆的if语句
*
*/
include("conn.php");
function clearcookies(){
    setCookie('username','',time()-3600);
    setCookie('logset','',time()-3600);
}
?>
  <?php
    if ($_POST['loginsubmit'])  //检查是否提交
    {
        clearcookies();
        $sql = "SELECT * FROM user WHERE username = '$_POST[username]'";
        $query = mysql_query($sql);
        $userinfo = mysql_fetch_array($query);
        if ($_POST[password] != "" & $_POST[username] != "") //检查用户名密码是否填写
        {
            if ($userinfo['password'] == $_POST['password'] ) //在SQL中检查用户名密码是否正确
            {
                $username = $_POST[username];
                
                setcookie('username','$username',time()+31536000,'/');
                setcookie('logset','ok',time()+31536000,'/');
                echo "<meta http-equiv=refresh content=0;url=index.php?foo=$_POST[username]>";
            }
            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,'/');