How to modify client-defined variable values through the server side (with Web interface)

How can I modify the value of a client-defined variable from the server side (with a web interface)?
Server is written by PHP, of course, including JS, etc., the client is written in C language on the device.

PS: Actually is the Authpuppy authentication server and writes on the router the WiFiDog.

What you want to do now is set an option on the server-side interface that saves the value to a variable on the client, assuming that the variable is: Timeout.

Do you have any good methods or ideas?
------Solution--------------------
This will see if your client supports duplex communication.
Usually the single work is relatively simple (client requests, the server returns data), the client does not need to be so complicated. It's just bad real time.
Duplex communication is actually done on the client monitor function (server-side request, the client returns data), real-time, but the cost is much higher

No matter what the language, just follow the agreement, there will be no problem

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