// SQL Query mein ID select karein
$sql = "SELECT id FROM users WHERE username = '$username' AND password = '$password'";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    $row = $result->fetch_assoc();
    
    // Session start karke ID save karein
    session_start();
    $_SESSION['user_id'] = $row['id']; 
    
    header("Location: dashboard.php");
}