Purpose: Marks the current session as ended and calculates its duration.

function endUserSession($pdo)
{
    if (!isset($_SESSION['session_id'])) {
        return false;
    }

    $session_id = $_SESSION['session_id'];

    $stmt = $pdo->prepare("
        UPDATE user_sessions
        SET
            session_end = NOW(),
            session_duration = TIMESTAMPDIFF(
                SECOND,
                session_start,
                NOW()
            )
        WHERE session_id = :session_id
    ");

    return $stmt->execute([
        'session_id' => $session_id
    ]);
}

1. Guard clause – If $_SESSION['session_id'] isn't set, there's nothing to end, so it returns false.
2. Reads the session ID from the PHP session.
3. Prepares an UPDATE on user_sessions:
session_end = current time (NOW()) session_duration = difference in seconds between session_start and NOW(), using MySQL's TIMESTAMPDIFF.
4. Filters by session_id so only the current row is updated.
5. Returns the result of execute() (a boolean: true on success, false on failure).