访问特定输入的Mysql用户到php

Question

我希望mysql用户的输入用户名和密码（可选）是用户而不是默认用户。如何改进此代码，它可以通知我已成功使用该用户并在用户不存在时发出警告。

<form class="box login" method="post">
<fieldset class="boxBody">
  <label>Username</label>
  <input type="text" tabindex="1" placeholder="Username" required name="username">
  <label><label class="rLink" tabindex="5">Optional</label>Password</label>
  <input type="password" tabindex="2" placeholder="Password" name="password" >
</fieldset>
<footer>

  <input type="submit" class="btnLogin" value="Login" tabindex="4" name="submit">
</footer>

<?php

if (isset($_POST['submit'])) {

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

$servername = "localhost";
$user = $username;
$pass = $password;

 if (empty($_POST['username'])) {
            echo  "Username can't be empty!"; }
    else {
        $conn = mysqli_connect($servername, $user, $pass);
            if (!$conn) {
                 die("Connection failed: " . mysqli_connect_error()); }
                    echo "Connected successfully"; }
}
?>