This my own class for private messaging system, built using plain PHP. It is still under development. To find out more, visit the issues page.
SimplePMS is a PHP message service with database backend using SQLite, PostgreSQL or MySQL
PHP API for Firebase Cloud Messaging from Google
Simplified real-time messaging package for Laravel 5.x.
Simple messaging system for Laravel
PHP API client for Scaledrone Realtime Messaging Service
A convenient way to handle messages between users in a simple way