login system

<?php
session_start();
include('dbconn.php');
$error_msg = "";
$sumbit = filter_input(INPUT_POST, 'submit');
if (isset($sumbit)) {
    $username = filter_input(INPUT_POST, 'username');
    $password = filter_input(INPUT_POST, 'password');


    if ($username != "" || $password != "") {

        $result = $db->query("SELECT `admin_login_id`, `admin_login_username`, `admin_login_status` FROM `acl_admin_login` WHERE `admin_login_username`='" . $username . "' and `admin_login_password`='" . md5($password) . "'");
        if ($result->num_rows > 0) {
            $row = $result->fetch_assoc();
            if ($row['admin_login_status'] == 1) {
                $_SESSION['name'] = $row['admin_login_username'];
                header("Location:aaaa.php");
            } else {
                $error_msg = "Sorry, we couldn't allow that username.";
            }
        } else {
            $error_msg = "username or password are wrong";
        }
    } else {
        $error_msg = "Please fill username & password";
    }
}
?>
<!DOCTYPE html>
<html>
    <head>
        <title>Testing</title>
        <link rel="shortcut icon" href="images/logo.png" />
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <link rel="stylesheet" href="css/bootstrap.min.css">
        <link href='https://fonts.googleapis.com/css?family=Asset|Diplomata+SC|Montserrat|Merriweather:400,900,700&subset=latin,latin-ext' rel='stylesheet' type='text/css'>
        <link rel="stylesheet" href="css/style.css">
    </head>
    <body>
        <div class="">
            <form class="form-signin col-lg-offset-3 col-lg-6" id="login-form" action="login.php"
                  method="POST">
                <h2 class="form-signin-heading">Login</h2>
                <label class="sr-only" for="inputUsername">Username</label>
                <input type="text" autofocus="" name="username" required="" oninvalid="this.setCustomValidity('Please insert your username')" placeholder="username" class="form-control" id="inputUsername">
                <label class="sr-only" for="inputPassword">Password</label>
                <input type="password" oninvalid="this.setCustomValidity('Please insert your password')" required="" name="password" placeholder="Password" class="form-control" id="inputPassword">
                <br>
                <button type="submit" class="btn btn-lg btn-primary btn-block submit" name="submit" >Sign in</button>
                <?php
                if ($error_msg != null || $error_msg != '') {
                    echo "<br><p style=\"color:red;\font-size:20px;\">" . $error_msg . "</p>";
                }
                ?>
            </form>
        </div>

        <script src="js/jquery-2.2.0.min.js"></script>
        <script src="js/bootstrap.min.js"></script>
        <script src="js/main.js"></script>
    </body>
</html>