php - Warning: Cannot modify header information - headers already sent by ERROR

Possible
Duplicate:

href="https://stackoverflow.com/questions/8028957/headers-already-sent-by-php">Headers
already sent by
PHP

I've
been struggling with this error for a while
now.

To start with, I just thought it was white
space, but after further research I think it might be a problem similar to
this:

Look for any statements that could send output to the user before this header
statement. If you find one or more, change your code to move the header statement before
them. Complex conditional statements may complicate the issue, but they may also help
solve the problem. Consider a conditional expression at the top of the PHP script that
determines the header value as early as possible and sets it
there.

I'm
guessing the include header is causing the problem along with the header(), but I'm not
sure how to rearrange the code to get rid of this
error.

How do I remove the
error?

            $username = $password = $token = $fName = "";



            include_once 'header.php';

 if (isset($_POST['username'])
            && isset($_POST['password']))
 $username =
            sanitizeString($_POST['username']);

 $password =
            sanitizeString($_POST['password']); //Set temporary username and password
            variables
 $token = md5("$password"); //Encrypt temporary
            password

 if ($username != 'admin')

            {

 header("Location:summary.php");
 }

            elseif($username == 'admin')
 {

            header("Location:admin.php");
 }
 elseif($username == '')

            {
 header("Location:index.php");
 }


            else
 die ("
            class='error'>Invalid username or
            password.
");

 if ($username ==
            "" || $token == "")
 {
 echo "            class='container'>
Please enter your username and
            password

";
}
else

{
$query = "SELECT * FROM members WHERE username='$username'AND password =
'$token'"; //Look in table for username entered

$result =
mysql_query($query);
if (!$result)
die ("Database access failed: "
. mysql_error());
elseif (mysql_num_rows($result) > 0)

{
$row = mysql_fetch_row($result);
$_SESSION['username'] =
$username; //Set session variables
$_SESSION['password'] =
$token;

$fName = $row[0];

}

}
?>