Hello.
I've clearly read your requirements and it's very interesting.
As a full stack web developer, i have much experience in building chat system.
The chat system can construct based on stream or request mode. First is based on websocket and Seconde is based on ajax.
You requires php-based chat system, so it will be ajax driven chat system.
I have sufficient experience with your job.
If you choose me, you will see my high skill and best quality.
Please contact me. Thanks.