php网站带数据库

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database_name";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检测连接
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}
echo "连接成功";

$stmt = $conn->prepare("SELECT * FROM users WHERE username = ?");
$stmt->bind_param("s", $username);
$username = $_POST['username'];
$stmt->execute();
$result = $stmt->get_result();

<form action="login.php" method="post">
  用户名: <input type="text" name="username"><br>
  密码: <input type="password" name="password"><br>
  <input type="submit" value="登录">
</form>

session_start();

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $username = $_POST['username'];
    $password = $_POST['password'];

    // 连接数据库
    $conn = new mysqli("localhost", "root", "", "testdb");
    if ($conn->connect_error) {
        die("连接失败: " . $conn->connect_error);
    }

    // 验证用户
    $sql = "SELECT id FROM users WHERE username='$username' AND password='$password'";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        $_SESSION["loggedin"] = true;
        $_SESSION["id"] = $result->fetch_assoc()["id"];
        header("location: welcome.php");
    } else {
        echo "用户名或密码错误";
    }
    $conn->close();
}