Mobile Php Members Website with Open-Source Messaging Like WHATSPP or LINE or VIBER

This should be almost same as SMS, except it is just sending within the site, so it should be 100% free. Because the messages don't go through and shall not go through any telcos. The most recently sent or received messages between any given user should help in proritizing contact list higher on the list of recent contacts, just as Whatsapp or Viber is doing. Also, make the message sending between users make it look like they are having a dialog. Messages sent and received never deleted. This should be fairly straightforward and simple to customize, except make it look like WHATSAPP or VIBER or LINE.