form login dengan fungsi

<!DOCtype>
<html>
<head>
    <title>Halaman login</title>
    <link rel="stylesheet" type="text/css" href="assets/css/style.css"> <!-- Ini digunakan untuk memanggil file css bootstrap   !-->
    <link rel="stylesheet" type="text/css" href="assets/css/bootstrap.css">
    <link rel="stylesheet" type="text/css" href="assets/css/bootstrap.min.css">
    <link rel="stylesheet" type="text/css" href="assets/css/bootstrap-theme.css">
    <link rel="stylesheet" type="text/css" href="assets/css/bootstrap-theme-min.css">
</head>
<body>
    <div class="col-md-4 col-md-offset-4 form-login">
        <div class="outter-form-login">
        <div class="logo-login">
            <center><img src="images/smk.png" width="50"/></center>
        </div>
        <br/>
        <br/>
        <?php
        if(isset($_GET['pesan']))
            {
            if($_GET['pesan'] == "gagal")
            {
                echo "<div class='alert alert-danger'>Gagal Login karena Username atau Password Salah</div>";
            }
            else if($_GET['pesan'] == "logout")
            {
                echo "<div class='alert alert-success'>Anda Berhasil Logout dari Sistem</div>";
            }else if($_GET['pesan'] == "belum_login")
            {
                echo "<div class='alert alert-warning'>Anda Belum Melakukan Login ke dalam Sistem</div>";
            }
        }
        ?>

            <form class="inner-login" method="post" action="proses_login.php">
            <h3 class="text-center title-login">Login ke dalam Aplikasi</h3>
                <div class="form-group">
                    <input type="text" class="form-control" name="username" placeholder="Masukkan Username Anda">
                </div>

                <div class="form-group">
                    <input type="password" class="form-control" name="password" placeholder="Masukkan Password Anda">
                </div>

                <button type="submit" class="btn btn-success">Login</button>
            </form>
        </div>
    </div>
</body>
    <script type="text/javascript" src="assets/js/bootstrap.js"></script> <!-- Ini digunakan untuk memanggil file javascript   !-->
    <script type="text/javascript" src="assets/js/bootstrap.min.js"></script>
    <script type="text/javascript" src="assets/js/npm.js"></script>
</html>