proses login

<?php
 session_start();
 include ('koneksi.php');

 $username = $_POST['username'];
 $password = $_POST['password'];

 $ambil = mysql_query("SELECT * FROM user WHERE username='$username' AND password='$password'");
 $ada = mysql_num_rows($ambil);
 $data = mysql_fetch_array($ada);

 if ($username == '' or $password =='') {
    ?>
    <script>
        alert(username dan passwor tidak boleh kosong);
        window.location.href'index.php';
    </script
 <?php
 }
 else if ($ada > 0){
    $_SESSION['username'] = $data['username'];
?>
    <script>
        window.location.href='menu.php';
    </script>
<?php
}
else {
?>
    <script>
        alert('Maaf, username dan Password anda salah !!!');
        window.location.href='index.php';
    </script>
<?php
}
?>