error

<?php
include("func.php");
 $username=@$_POST['username'];
 $userpass=@$_POST['userpass'];
$link=koneksi_db();
 $sql="select * from user where username='$username' and userpass=password('$userpass')";
$res=mysql_query($sql,$link);
if(mysql_num_rows($res)==1) // apabila username dan userpass benar
 {
 $data=mysql_fetch_array($res);
session_start();
//session_register("username","nama","level","sudahlogin");
$_SESSION['username']=$data['username']; // isi variabel username
 $_SESSION['nama']=$data['nama']; // isi vareiabel nama
$_SESSION['level']=$data['level']; // isi variabel level
 $_SESSION['sudahlogin']=true; // variabel status sudah login
header("Location:index.php"); // pindah ke halaman index.php
}
else
 {

?>
        <a href="index.php">
        <script>
        alert("username atau password salah!");
        document.location="index.php";
        </script>
        </a>
        <?php
 }

?>