Question

I`m making an little php server that has 2 clients, on connects to the php (device a) and controls the other (device b).

device a makes a get request to the php. now i want to push that command to device b. is there any way to echo to device b? or to make an request to the device b?

(i only need to send one character to device b)

Answer 1

Pushing to device is possible but depends on your device.
A solution would be websockets, see the following links for further reading:

http://www.websocket.org/

http://code.google.com/p/phpwebsocket/

Another solution would be longpolling which is easy to implement in php:
http://en.wikipedia.org/wiki/Push_technology#Long_polling

Very simple implementation of longpolling on server-side:

$ts = time();
while(true) {
    // if there's something new, send the response to the server
    // if not, continue with the loop
    if($response = getSuperAwesomeResponse()) {
        print $response;
        break;
    }

    // timeout after 60 seconds
    if( ($ts + 60) > time()) {
        break;
    }

    sleep(1);
}

On the client-side you just need to send some sort of ajax calls