Android One to One Messaging App Using Ratchet Websocket

Question

I want to create a real time chat app like whatsapp for android using php and ratchet websocket. Every tutorial shows how to use it with group chat (as below code) but I want to use it for private messages either. Is there any way to do this for private chat? If yes, how can I do?

<?php
namespace MyApp;
use Ratchet\MessageComponentInterface;
use Ratchet\ConnectionInterface;

class Chat implements MessageComponentInterface {

    private $clients;
    private $users;

    public function __construct()
    {
        $this->clients = array();
        $this->users = array();
    }

    public function onOpen(ConnectionInterface $conn) {
        $this->clients[] = $conn;

        echo "New Connection";
    }

    public function onMessage(ConnectionInterface $from, $msg) {
        foreach ($this->clients as $client){
            if ($client != $from){
                $client->send($msg); 
            }
        }
    }

    public function onClose(ConnectionInterface $conn) {
        echo "Connection closed";
    }

    public function onError(ConnectionInterface $conn, \Exception $e) {
    }
}