///// PHP /////

db = MySQLlibrary.connect(yourDetailsGoHere);

sock = socket_create(socket details, go here);

socket_listen(sock, OtherSettingsHere);

do {
  data = sock.read();
  if (data == 'r'){
    sock.send(db.doShit());
  }
} while (1);


///// Arduino /////

void setup(){
  Ethernet.begin(yourDetails);
  client.connect(server, port);
}

void loop(){
  client.print("r");
  delay(100);

  while(client.available()){
    someVariable = client.read();
  }
}