FOSMessage is a PHP 5.4+ framework-agnostic library providing a data structure and common features to set up user-to-user messaging systems.
You can think of it as a model for your messaging features : it will take care of the consistency of the data for you in order to easily create a full-featured messaging system.
Note : This library is currently in development. You can test it in your project (the Composer installation process is very simple), but you should not use it in production for the moment.
This library is based on concepts shared by most modern frameworks (dependency injection, event dispatching, abstract data drivers, etc.) and therefore, it’s very easy to set it up in any kind of context.
If you want to set it up in Symfony, FOSMesageBundle is being developed in a new version (not ready yet).
You can read the documentation here.
An implementation example is available on Github: tgalopin/FOSMessage-demo.