I'll provide a simple example of a web application using PHP. This example will demonstrate a basic registration and login system. For brevity, I won't go into detail about securing every aspect of the application, but I'll point out some important security practices.

Create a file called `config.php` for your database connection:

```php
<?php
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "your_database_name";

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

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
?>
```

Create a file called `register.php` for user registration:

```php
<?php
include('config.php');

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $username = filter_var($_POST["username"], FILTER_SANITIZE_STRING);
    $email = filter_var($_POST["email"], FILTER_SANITIZE_EMAIL);
    $password = $_POST["password"];

    if (empty($username) || empty($email) || empty($password)) {
        echo "All fields are required.";
    } else {
        $hashed_password = password_hash($password, PASSWORD_DEFAULT);

        $stmt = $conn->prepare("INSERT INTO users (username, email, password) VALUES (?, ?, ?)");
        $stmt->bind_param("sss", $username, $email, $hashed_password);

        if ($stmt->execute()) {
            echo "User registered successfully.";
        } else {
            echo "Error: " . $stmt->error;
        }

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

<!DOCTYPE html>
<html>
<head>
    <title>Registration</title>
</head>
<body>
    <h1>Register</h1>
    <form action="register.php" method="post">
        <input type="text" name="username" placeholder="Username" required>
        <input type="email" name="email" placeholder="Email" required>
        <input type="password" name="password" placeholder="Password" required>
        <input type="submit" value="Register">
    </form>
</body>
</html>
```

Create a file called `login.php` for user login:

```php
<?php
include('config.php');
session_start();

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

    if (empty($username) || empty($password)) {
        echo "All fields are required.";
    } else {
        $stmt = $conn->prepare("SELECT id, username, password FROM users WHERE username = ?");
        $stmt->bind_param("s", $username);

        if ($stmt->execute()) {
            $stmt->store_result();
            $stmt->bind_result($id, $username, $hashed_password);

            if ($stmt->num_rows > 0) {
                $stmt->fetch();
                if (password_verify($password, $hashed_password)) {
                    $_SESSION["loggedin"] = true;
                    $_SESSION["id"] = $id;
                    $_SESSION["username"] = $username;
                    header("location: welcome.php");
                } else {
                    echo "Invalid username or password.";
                }
            } else {
                echo "Invalid username or password.";
            }
        } else {
            echo "Error: " . $stmt->error;
        }

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

<!DOCTYPE html>
<html>
<head>
    <title>Login</title>
</head>
<body>
    <h1>Login</h1>
    <form action="login.php" method="post">
        <input type="text" name="username" placeholder="Username" required>
        <input type="password" name="password" placeholder="Password" required>
        <input type="submit" value="Login">
    </form>
</body>
</html>
```

Create a file called `welcome.php` for the welcome page after a successful login:

```php
<?php
session_start();

if (!isset($_SESSION["loggedin"]) || $_SESSION["loggedin"] !== true) {
    header("location: login.php");
    exit;
}
?>

<!DOCTYPE html>
<html>
<head>
    <title>Welcome</title>
</head>
<body>
    <h1>Welcome, <?php echo htmlspecialchars($_SESSION["username"]); ?>!</h1>
    <p><a href="logout.php">Logout</a></p>
</body>
</html>
```

Create a file called `logout.php` for user logout:

```php
<?php
session_start();
$_SESSION = array();
session_destroy();
header("location: login.php");
exit;
?>
```

Make sure to create a users table in your MySQL database with the following structure:

```sql
CREATE TABLE users (
  id INT(11) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(50) NOT NULL UNIQUE,
  email VARCHAR(50) NOT NULL UNIQUE,
  password VARCHAR(255) NOT NULL
);
```

Please note that this example is a simple illustration of a PHP web application and may not be fully secure. For production use, it's crucial to follow security best practices, including validating and sanitizing user input, using prepared statements, and implementing proper session management, as mentioned in the previous answer.
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