Nhập viện với bệnh Cannot modify header information của PHP-MySql

<?php
//ob_start();
//ob_end_clean();
// Connects to your Database 
require_once("configuration.php");
//Checks if there is a login cookie
if(isset($_COOKIE['ID_my_site']))
//if there is, it logs you in and directes you to the members page
{ 
	$username = $_COOKIE['ID_my_site']; 
	$pass = $_COOKIE['Key_my_site'];
	$check = mysql_query("SELECT * FROM users WHERE username = '$username'")or die(mysql_error());
	while($info = mysql_fetch_array( $check )) 	
		{
		if ($pass != $info['password']) 
			{
			}
		else
			{
			header("Location: members.php");
			}
		}
}
//if the login form is submitted
if (isset($_POST['submit'])) { // if form has been submitted
// makes sure they filled it in
	if(!$_POST['username'] | !$_POST['pass']) {
		die('You did not fill in a required field.');
	}
	// checks it against the database
	if (!get_magic_quotes_gpc()) {
		$_POST['email'] = addslashes($_POST['email']);
	}
	$check = mysql_query("SELECT * FROM users WHERE username = '".$_POST['username']."'")or die(mysql_error());
//Gives error if user dosen't exist
$check2 = mysql_num_rows($check);
if ($check2 == 0) {
		die('That user does not exist in our database. <a href=register.php>Click Here to Register</a>');
				}
while($info = mysql_fetch_array( $check )) 	
{
$_POST['pass'] = stripslashes($_POST['pass']);
	$info['password'] = stripslashes($info['password']);
	$_POST['pass'] = md5($_POST['pass']);
//gives error if the password is wrong
	if ($_POST['pass'] != $info['password']) {
		die('Incorrect password, please try again.');
	}
else
{
// if login is ok then we add a cookie 
$_POST['username'] = stripslashes($_POST['username']);
$hour = time() + 3600; 
setcookie(ID_my_site, $_POST['username'], $hour);
setcookie(Key_my_site, $_POST['pass'], $hour);	
//then redirect them to the members area
header("Location: members.php");
}
}
} else {	
// if they are not logged in
require_once("header.php");
?>
<form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">
<table border="0">
<tr><td colspan=2><h1>Login</h1></td></tr>
<tr><td>Tên truy cập:</td><td>
<input class="inputbox" type="text" name="username" maxlength="40">
</td></tr>
<tr><td>Mật khẩu:</td><td>
<input class="inputbox" type="password" name="pass" maxlength="50">
</td></tr>
<tr><td colspan="2" align="right">
<input class="button" type="submit" name="submit" value="Đồng ý">
<input class="button" type="reset" name="clean" value="Huỷ bỏ">
</td></tr>
</table>
</form>
<?php
require_once("footer.php");
}
?>