php登录程序

<?php
// 数据库连接
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 用户输入
$user = $_POST['username'];
$pass = $_POST['password'];

// 查询语句
$sql = "SELECT * FROM users WHERE username='$user' AND password=MD5('$pass')";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 用户名和密码正确
    session_start();
    $_SESSION['username'] = $user;
    header("Location: dashboard.php");
} else {
    echo "用户名或密码错误";
}

$conn->close();
?>

<?php
session_start();

if (isset($_SESSION['username'])) {
    echo "欢迎回来, " . $_SESSION['username'] . "!";
} else {
    header("Location: login.php");
}
?>

<?php
// 预处理语句防止SQL注入
$stmt = $conn->prepare("SELECT * FROM users WHERE username=? AND password=MD5(?)");
$stmt->bind_param("ss", $user, $pass);
$stmt->execute();
$result = $stmt->get_result();

if ($result->num_rows > 0) {
    // 用户名和密码正确
    session_start();
    $_SESSION['username'] = $user;
    header("Location: dashboard.php");
} else {
    echo "用户名或密码错误";
}

$stmt->close();
$conn->close();
?>