Send messages to users and reply to them (NOT by mail)?

Question

PHP

Good afternoon! Please tell me, there is a task: The client places an order -> Information about the order is sent to the executors -> The executors send a response via input -> Then these sent values ​​are compared and sent to the client. In what area to start digging or are there any ready-made examples? The client and performers have their own page of one common site (Example site.com/client.html | site.com/worker.html) I would be grateful for any help!