写一个简约的后台登陆界面用php

<?php
session_start();
require('config.php');

if(isset($_POST['submit'])){
    $username = $_POST['username'];
    $password = $_POST['password'];
    $stmt = $pdo->prepare("SELECT * FROM users WHERE username=? AND password=?");
    $stmt->execute([$username, $password]);
    $user = $stmt->fetch();

    if($user){
        $_SESSION['username'] = $user['username'];
        $_SESSION['role'] = $user['role'];
        header('Location: dashboard.php');
        exit();
    }else{
        $error = "用户名或密码错误";
    }
}
?>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>后台登陆界面</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
    <div class="container">
        <form action="login.php" method="post">
            <h2>后台登陆</h2>
            <?php if(isset($error)): ?>
            <p class="error"><?php echo $error ?></p>
            <?php endif; ?>
            <div class="form-control">
                <label for="username">用户名</label>
                <input type="text" id="username" name="username">
            </div>
            <div class="form-control">
                <label for="password">密码</label>
                <input type="password" id="password" name="password">
            </div>
            <button type="submit" name="submit">登陆</button>
        </form>
    </div>
</body>
</html>