Katbin

<?php
    //session_start();
    require_once 'config.php';
?>
-----
<?php
        if (isset($_POST['login'])) {
            
            $username = $_POST["username"];
            $password = $_POST["password"];

            $sql = "SELECT id, username FROM users 
                    WHERE 
                    username = '$username' AND upassword = '$password'";
            
            $result = $conn->query($sql);

            if ($result ->num_rows > 0) {
                session_start(); // you can add this at the start line of the document
                
                $row = $result -> fetch_assoc();
                $_SESSION["user_id"] = $row["id"];
                $_SESSION["user_name"] = $row["username"];
                
                header("Location: dashboard.php");
            } 
            else {
                echo '<p class="error-message">Invalid username or password.</p>';
            }
            $conn->close();
        }
    ?>
-----
<form action="login.php" method="post">