html5表单不能用php提交也不能对mysql表执行操作

<form action="submit.php" method="post">
  <input type="text" name="username" placeholder="用户名">
  <input type="password" name="password" placeholder="密码">
  <input type="submit" value="提交">
</form>

<?php
// 获取表单数据
$username = $_POST['username'];
$password = $_POST['password'];

// 连接到MySQL数据库
$servername = "数据库服务器地址";
$dbname = "数据库名";
$username_db = "数据库用户名";
$password_db = "数据库密码";

$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username_db, $password_db);

// 执行插入操作
$stmt = $conn->prepare("INSERT INTO users (username, password) VALUES (:username, :password)");
$stmt->bindParam(':username', $username);
$stmt->bindParam(':password', $password);
$stmt->execute();

// 执行查询操作
$stmt = $conn->prepare("SELECT * FROM users");
$stmt->execute();
$result = $stmt->fetchAll(PDO::FETCH_ASSOC);

// 输出查询结果
foreach ($result as $row) {
  echo "用户名: " . $row['username'] . "<br>";
  echo "密码: " . $row['password'] . "<br>";
}

// 关闭数据库连接
$conn = null;
?>