Cannot modify header information - headers already sent by

[ARy]

<?php 
	session_start();
	require_once('db_login.php');
	require_once('DB.php');
	
	if (empty($_SESSION['user_id']))
	{
                  /*第一个header（）函数总是报错*/
		if (!isset($_SERVER['PHP_AUTH_USER']) || !isset($_SERVER['PHP_AUTH_PW']))
		{
			header('WWW-Authenticate: Basic realm = "Member Area"');
			header("HTTP/1.0 401 Unauthorized");
			echo "You must enter a username and password combination!";
			exit;
		}
	}	
	
	$connection = DB::connect("mysql://$db_username:$db_password@$db_host/$db_database");
	if (DB::isError($connection))
	{
		echo "Could not connect to the data base:<br />".DB::errorMessage($connection);
	}
	
	$username = mysql_real_escape_string($_SERVER['PHP_AUTH_USER']);
	$password = mysql_real_escape_string($_SERVER['PHP_AUTH_USER']);
	$query = "select user_id, username from users where username = '".$username."' and  password = MD5('".$password."') limit 1";
	$result = $connection->query($query);
	
	if (!$row = $result->fetchRow(DB_FETCHMODE_ASSOC))
	{/*第二个header()*/
		header('WWW-Authenticate: Basic realm = "Member Area"');
		header("HTTP/1.0 401 Unauthorized");
		echo "Your username and password combination was incorrect!";
		exit;
	}
	
	$_SESSION['user_id'] = $row['user_id'];
	$_SESSION['username'] = $row['username'];
	
	echo "You have successfully logged in as: ".$_SESSION["username"].".";
	
?>

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\db_login.php:7) in C:\xampp\htdocs\user_login_main.php on line 10

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\db_login.php:7) in C:\xampp\htdocs\user_login_main.php on line 11
You must enter a username and password combination!

这个一个用户登录验证
查资料说header()要放在最前面，但是尽管可以吧第一次的header()放在最前面，但是我的第二个header()仍然会报错，求高手求救该怎么改？

[kyzy_yy_pm]

第二个header之前有输出

要注意：header之前不能有任何输出(包括空格标签等都属于输出)、错误

[lazyboy_wu]

<?php 
    ob_statr();
    session_start();
    require_once('db_login.php');
    require_once('DB.php');
    
    if (empty($_SESSION['user_id']))
    {
                  /*第一个header（）函数总是报错*/
        if (!isset($_SERVER['PHP_AUTH_USER']) || !isset($_SERVER['PHP_AUTH_PW']))
        {
            header('WWW-Authenticate: Basic realm = "Member Area"');
            header("HTTP/1.0 401 Unauthorized");
            ob_clean_flush();
            echo "You must enter a username and password combination!";
            exit;
        }
    }    
    ob_clean_flush();

[ImN1]

[Quote=引用 2 楼 hnxxwyq 的回复:]

PHP code


<?php
ob_statr();
session_start();
require_once('db_login.php');
require_once('DB.php');

if (empty($_SESSION['user_id']))
{
/*第一个header（）函数……
[/Quote]

原因是？require里面的问题？