Create an API in a ready-made php project

There is a ready-made system written in php to connect gsm pools
Equipment on the client side - Single-board orange pi gateway.
Data is transmitted to the server
All interaction takes place through the web interface on the server,
Which transmits/receives commands from Orange.
Simbank is connected to the orange via USB.

At the moment, the web interface shows all the equipment that is connected and only one user.
And Orange writes directly to the database on the server.
Task of reconfiguring the client-server bundle on the API
And make it so that each user saw only his own equipment on the server.

Please write your terms and amount.
We also need further support for this project.
We are ready to answer questions.