What's wrong? It can't define what stands in this title, and the textbox's name sent HAS the name username

PHP Code:
<?php
    include('config.php');
    // connect to the mysql database server.
    mysql_connect ($dbhost, $dbusername, $dbuserpass);
    mysql_select_db($dbname) or die('Cannot select database');
    if ( isset( $_COOKIE['SundhedU'] ) && isset( $_COOKIE['SundhedP'] ) )
    {
        header( "Location: main.php" );
    }
    else
    {
    if($_POST['username'] {
            //did they supply a password and username
            $username=$_POST['username'];
            $password=$_POST['password'];
            if ($password==NULL) { 
echo "<META HTTP-EQUIV='REFRESH' CONTENT='0;URL=index.php'>
";
}
            else
            {
                $query = mysql_query("SELECT username,password FROM users WHERE username = '$username'") or die(mysql_error());
                $data = mysql_fetch_array($query);
                if($data['password'] != md5($password))
                { 
echo "<META HTTP-EQUIV='REFRESH' CONTENT='0;URL=index.php'>";
}
                else
                {
                    if (isset($_POST['husklogin']))
                    {
                        setcookie('SundhedU', $username, time() + 3600 * 24 * 365);
                        setcookie('SundhedP', md5($password), time() + 3600 * 24 * 365);       
      
                        $query = mysql_query("SELECT username,password FROM users WHERE username = '$username'") or die(mysql_error());
                        $row = mysql_fetch_array($query);
                        $_SESSION["s_username"] = $row['username'];
                        header("Location: index.php");
                    }else{
                        $query = mysql_query("SELECT username,password FROM users WHERE username = '$username'") or die(mysql_error());
                        $row = mysql_fetch_array($query);
                        $_SESSION["s_username"] = $row['username'];
                        header("Location: index.php");
}
                }
            }
        }   
        }
?>