php后台登录

<?php
$username = $_POST['username'];
$password = $_POST['password'];

// 假设数据库连接代码已经完成
$sql = "SELECT * FROM users WHERE username = '$username'";
$result = mysqli_query($conn, $sql);

if (mysqli_num_rows($result) > 0) {
    $row = mysqli_fetch_assoc($result);
    if (password_verify($password, $row['password'])) {
        // 登录成功
        session_start();
        $_SESSION['user_id'] = $row['id'];
        header('Location: dashboard.php');
        exit();
    } else {
        echo "密码错误";
    }
} else {
    echo "用户名不存在";
}
?>

<?php
session_start();

if (isset($_SESSION['user_id'])) {
    // 用户已登录
    echo "欢迎回来，用户ID：" . $_SESSION['user_id'];
} else {
    // 用户未登录
    header('Location: login.php');
    exit();
}
?>

<?php
$username = mysqli_real_escape_string($conn, $_POST['username']);
$password = mysqli_real_escape_string($conn, $_POST['password']);

$sql = "SELECT * FROM users WHERE username = '$username'";
$result = mysqli_query($conn, $sql);
// 其他代码...
?>