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PHP LogoUser Registration and Login System

A User Registration and Login System is a fundamental component of many web applications, enabling users to create unique accounts, authenticate their identity, and access personalized content or restricted areas. It's crucial for user management, personalization, and security.

Core Components:

1. Registration:
* Data Collection: Gathers user information (e.g., username, email, password) via an HTML form.
* Input Validation: Ensures the data meets specific criteria (e.g., strong password, valid email format, unique username/email) to maintain data integrity and prevent common vulnerabilities.
* Password Hashing: Instead of storing passwords in plain text, which is a major security risk, passwords are cryptographically hashed using a one-way function (e.g., `password_hash()` in PHP). This hash is then stored in the database. If the database is compromised, the actual passwords remain secure.
* Database Storage: The validated and hashed user data is stored in a database (e.g., MySQL).

2. Login:
* Credential Input: Users provide their identifying credentials (e.g., username/email and password) through an HTML form.
* Authentication: The system retrieves the user's stored hash from the database based on the provided identifier. The submitted password is then hashed using the same algorithm, and this new hash is compared with the stored hash using a secure function (e.g., `password_verify()` in PHP). This comparison verifies the password without ever needing to decrypt the stored hash.
* Session Management: Upon successful authentication, a server-side session is initiated. A unique session ID is typically stored in a cookie on the user's browser, and corresponding user data (like user ID, username) is stored on the server (`$_SESSION` in PHP). This allows the application to remember the user's logged-in state across multiple page requests without requiring re-authentication.
* Authorization: After login, the system can determine what resources or functionalities the authenticated user is permitted to access.

3. Logout:
* Session Termination: When a user logs out, the server-side session data associated with their session ID is destroyed (`session_unset()`, `session_destroy()` in PHP), invalidating their logged-in state. The session cookie is also typically removed or expired.
* Redirection: The user is typically redirected to a public page (e.g., login page or homepage).

Security Considerations:
* Password Hashing: Always use strong, modern hashing algorithms like bcrypt (default for `password_hash()` in PHP).
* Input Validation & Sanitization: Prevent SQL injection, XSS (Cross-Site Scripting), and other attacks by properly validating and sanitizing all user inputs.
* Secure Sessions: Use HTTPS to encrypt communication, set `httponly` and `secure` flags on session cookies to mitigate XSS and ensure cookies are only sent over secure connections.
* Error Handling: Avoid revealing too much information in error messages (e.g., 'username does not exist' vs. 'invalid credentials').
* Rate Limiting: Implement measures to prevent brute-force attacks on login forms by limiting login attempts.

Technologies Involved (Example):
* HTML: For creating the registration and login forms.
* PHP: For handling server-side logic, processing form submissions, interacting with the database, and managing sessions.
* MySQL (or other RDBMS): For storing user data (usernames, hashed passwords, emails, etc.).
* CSS/JavaScript: For styling and enhancing the user interface (optional but common).

Example Code

```php
-- SQL Schema for 'users' table (create this in your database first)
-- Database: user_system
-- Table: users
CREATE DATABASE IF NOT EXISTS user_system;
USE user_system;

CREATE TABLE IF NOT EXISTS users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL UNIQUE,
    email VARCHAR(100) NOT NULL UNIQUE,
    password VARCHAR(255) NOT NULL,
    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
);

-- File: config.php (Database connection settings)
<?php
// Database credentials
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root'); // CHANGE THIS TO YOUR DATABASE USERNAME
define('DB_PASSWORD', '');     // CHANGE THIS TO YOUR DATABASE PASSWORD
define('DB_NAME', 'user_system');

// Attempt to connect to MySQL database
try {
    $pdo = new PDO("mysql:host=" . DB_SERVER . ";dbname=" . DB_NAME, DB_USERNAME, DB_PASSWORD);
    // Set the PDO error mode to exception
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    die("ERROR: Could not connect. " . $e->getMessage());
}
?>

-- File: register.php (User Registration Form and Logic)
<?php
session_start();
require_once 'config.php';

// Redirect to dashboard if already logged in
if (isset($_SESSION['user_id'])) {
    header('Location: dashboard.php');
    exit;
}

// Define variables and initialize with empty values
$username = $email = $password = $confirm_password = '';
$username_err = $email_err = $password_err = $confirm_password_err = '';

// Processing form data when form is submitted
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    // Validate username
    if (empty(trim($_POST['username']))) {
        $username_err = 'Please enter a username.';
    } elseif (!preg_match('/^[a-zA-Z0-9_]+$/', trim($_POST['username']))) {
        $username_err = 'Username can only contain letters, numbers, and underscores.';
    } else {
        // Prepare a select statement
        $sql = 'SELECT id FROM users WHERE username = :username';
        if ($stmt = $pdo->prepare($sql)) {
            $stmt->bindParam(':username', $param_username, PDO::PARAM_STR);
            $param_username = trim($_POST['username']);
            if ($stmt->execute()) {
                if ($stmt->rowCount() == 1) {
                    $username_err = 'This username is already taken.';
                } else {
                    $username = trim($_POST['username']);
                }
            } else {
                echo 'Oops! Something went wrong. Please try again later.';
            }
            unset($stmt);
        }
    }

    // Validate email
    if (empty(trim($_POST['email']))) {
        $email_err = 'Please enter an email address.';
    } elseif (!filter_var(trim($_POST['email']), FILTER_VALIDATE_EMAIL)) {
        $email_err = 'Please enter a valid email address.';
    } else {
        // Prepare a select statement
        $sql = 'SELECT id FROM users WHERE email = :email';
        if ($stmt = $pdo->prepare($sql)) {
            $stmt->bindParam(':email', $param_email, PDO::PARAM_STR);
            $param_email = trim($_POST['email']);
            if ($stmt->execute()) {
                if ($stmt->rowCount() == 1) {
                    $email_err = 'This email is already registered.';
                } else {
                    $email = trim($_POST['email']);
                }
            } else {
                echo 'Oops! Something went wrong. Please try again later.';
            }
            unset($stmt);
        }
    }

    // Validate password
    if (empty(trim($_POST['password']))) {
        $password_err = 'Please enter a password.';
    } elseif (strlen(trim($_POST['password'])) < 6) {
        $password_err = 'Password must have at least 6 characters.';
    } else {
        $password = trim($_POST['password']);
    }

    // Validate confirm password
    if (empty(trim($_POST['confirm_password']))) {
        $confirm_password_err = 'Please confirm password.';
    } else {
        $confirm_password = trim($_POST['confirm_password']);
        if (empty($password_err) && ($password != $confirm_password)) {
            $confirm_password_err = 'Password did not match.';
        }
    }

    // Check input errors before inserting into database
    if (empty($username_err) && empty($email_err) && empty($password_err) && empty($confirm_password_err)) {
        // Prepare an insert statement
        $sql = 'INSERT INTO users (username, email, password) VALUES (:username, :email, :password)';
        if ($stmt = $pdo->prepare($sql)) {
            $stmt->bindParam(':username', $param_username, PDO::PARAM_STR);
            $stmt->bindParam(':email', $param_email, PDO::PARAM_STR);
            $stmt->bindParam(':password', $param_password, PDO::PARAM_STR);

            // Set parameters
            $param_username = $username;
            $param_email = $email;
            $param_password = password_hash($password, PASSWORD_DEFAULT); // Creates a password hash

            // Attempt to execute the prepared statement
            if ($stmt->execute()) {
                // Redirect to login page
                header('Location: login.php');
                exit;
            } else {
                echo 'Something went wrong. Please try again later.';
            }
            unset($stmt);
        }
    }
    // Close connection
    unset($pdo);
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Register</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <style>
        body { font: 14px sans-serif; }
        .wrapper { width: 360px; padding: 20px; margin: auto; margin-top: 50px; border: 1px solid #ddd; border-radius: 5px; }
        .error-message { color: red; font-size: 0.9em; margin-top: 5px; }
    </style>
</head>
<body>
    <div class="wrapper">
        <h2>Register</h2>
        <p>Please fill this form to create an account.</p>
        <form action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']); ?>" method="post">
            <div class="form-group">
                <label>Username</label>
                <input type="text" name="username" class="form-control <?php echo (!empty($username_err)) ? 'is-invalid' : ''; ?>" value="<?php echo htmlspecialchars($username); ?>">
                <span class="error-message"><?php echo $username_err; ?></span>
            </div>
            <div class="form-group">
                <label>Email</label>
                <input type="email" name="email" class="form-control <?php echo (!empty($email_err)) ? 'is-invalid' : ''; ?>" value="<?php echo htmlspecialchars($email); ?>">
                <span class="error-message"><?php echo $email_err; ?></span>
            </div>
            <div class="form-group">
                <label>Password</label>
                <input type="password" name="password" class="form-control <?php echo (!empty($password_err)) ? 'is-invalid' : ''; ?>" value="<?php echo htmlspecialchars($password); ?>">
                <span class="error-message"><?php echo $password_err; ?></span>
            </div>
            <div class="form-group">
                <label>Confirm Password</label>
                <input type="password" name="confirm_password" class="form-control <?php echo (!empty($confirm_password_err)) ? 'is-invalid' : ''; ?>" value="<?php echo htmlspecialchars($confirm_password); ?>">
                <span class="error-message"><?php echo $confirm_password_err; ?></span>
            </div>
            <div class="form-group">
                <input type="submit" class="btn btn-primary" value="Register">
                <input type="reset" class="btn btn-secondary ml-2" value="Reset">
            </div>
            <p>Already have an account? <a href="login.php">Login here</a>.</p>
        </form>
    </div>
</body>
</html>

-- File: login.php (User Login Form and Logic)
<?php
session_start();
require_once 'config.php';

// Redirect to dashboard if already logged in
if (isset($_SESSION['user_id'])) {
    header('Location: dashboard.php');
    exit;
}

// Define variables and initialize with empty values
$email = $password = '';
$email_err = $password_err = $login_err = '';

// Processing form data when form is submitted
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    // Validate email
    if (empty(trim($_POST['email']))) {
        $email_err = 'Please enter your email.';
    } else {
        $email = trim($_POST['email']);
    }

    // Validate password
    if (empty(trim($_POST['password']))) {
        $password_err = 'Please enter your password.';
    } else {
        $password = trim($_POST['password']);
    }

    // Check for errors before attempting to log in
    if (empty($email_err) && empty($password_err)) {
        // Prepare a select statement
        $sql = 'SELECT id, username, email, password FROM users WHERE email = :email';
        if ($stmt = $pdo->prepare($sql)) {
            $stmt->bindParam(':email', $param_email, PDO::PARAM_STR);
            $param_email = $email;

            // Attempt to execute the prepared statement
            if ($stmt->execute()) {
                // Check if email exists, if yes then verify password
                if ($stmt->rowCount() == 1) {
                    if ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
                        $id = $row['id'];
                        $username = $row['username'];
                        $hashed_password = $row['password'];

                        if (password_verify($password, $hashed_password)) {
                            // Password is correct, start a new session
                            session_regenerate_id(true); // Regenerate session ID for security
                            $_SESSION['user_id'] = $id;
                            $_SESSION['username'] = $username;
                            $_SESSION['email'] = $email;

                            // Redirect user to dashboard page
                            header('Location: dashboard.php');
                            exit;
                        } else {
                            // Password is not valid, display a generic error message
                            $login_err = 'Invalid email or password.';
                        }
                    }
                } else {
                    // Email doesn't exist, display a generic error message
                    $login_err = 'Invalid email or password.';
                }
            } else {
                echo 'Oops! Something went wrong. Please try again later.';
            }
            unset($stmt);
        }
    }
    // Close connection
    unset($pdo);
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Login</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <style>
        body { font: 14px sans-serif; }
        .wrapper { width: 360px; padding: 20px; margin: auto; margin-top: 50px; border: 1px solid #ddd; border-radius: 5px; }
        .error-message { color: red; font-size: 0.9em; margin-top: 5px; }
    </style>
</head>
<body>
    <div class="wrapper">
        <h2>Login</h2>
        <p>Please fill in your credentials to login.</p>

        <?php
        if (!empty($login_err)) {
            echo '<div class="alert alert-danger">' . $login_err . '</div>';
        }
        ?>

        <form action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']); ?>" method="post">
            <div class="form-group">
                <label>Email</label>
                <input type="text" name="email" class="form-control <?php echo (!empty($email_err)) ? 'is-invalid' : ''; ?>" value="<?php echo htmlspecialchars($email); ?>">
                <span class="error-message"><?php echo $email_err; ?></span>
            </div>
            <div class="form-group">
                <label>Password</label>
                <input type="password" name="password" class="form-control <?php echo (!empty($password_err)) ? 'is-invalid' : ''; ?>">
                <span class="error-message"><?php echo $password_err; ?></span>
            </div>
            <div class="form-group">
                <input type="submit" class="btn btn-primary" value="Login">
            </div>
            <p>Don't have an account? <a href="register.php">Sign up now</a>.</p>
        </form>
    </div>
</body>
</html>

-- File: dashboard.php (Protected page for logged-in users)
<?php
session_start();

// Check if the user is logged in, if not then redirect to login page
if (!isset($_SESSION['user_id'])) {
    header('Location: login.php');
    exit;
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Dashboard</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <style>
        body { font: 14px sans-serif; text-align: center; }
        .wrapper { width: 600px; padding: 20px; margin: auto; margin-top: 50px; border: 1px solid #ddd; border-radius: 5px; }
    </style>
</head>
<body>
    <div class="wrapper">
        <h1>Welcome, <b><?php echo htmlspecialchars($_SESSION['username']); ?></b>!</h1>
        <p>This is your personalized dashboard. You are logged in.</p>
        <p>
            <a href="logout.php" class="btn btn-danger">Sign Out of Your Account</a>
        </p>
    </div>
</body>
</html>

-- File: logout.php (Handles user logout)
<?php
session_start();

// Unset all of the session variables
$_SESSION = array();

// Destroy the session. This will delete the session file on the server.
session_destroy();

// Redirect to login page
header('Location: login.php');
exit;
?>
```

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