<?php

// Connect to the MySQL database
$db = mysql_connect('localhost', 'root', 'password');
if (!$db) {
    die('Could not connect: ' . mysql_error());
}

// Select the database
mysql_select_db('users', $db);

// Check if the form has been submitted
if (isset($_POST['submit'])) {
    // Escape the user input to prevent SQL injection attacks
    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);

    // Query the database to check if the user exists
    $query = "SELECT * FROM users WHERE username='$username' AND password='$password'";
    $result = mysql_query($query);

    // If a result is returned, the user is logged in
    if (mysql_num_rows($result) == 1) {
        // Start a session and redirect to the dashboard
        session_start();
        $_SESSION['username'] = $username;
        header('Location: dashboard.php');
    } else {
        // If the user doesn't exist, display an error message
        echo "Invalid username or password.";
    }
}

?>