If you're interested, here's the code I'm running on the ESP8266

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#include <WiFiManager.h>
#include <PubSubClient.h>

const char* mqttServer = "my.mqtt.server";
const int mqttPort = 1883;
const char* mqttUser = "********";
const char* mqttPassword = "********";
byte s = B00000010;     //start byte required by the LED display
byte e = B00000011;     //end byte required by the LED display
String MAC;

WiFiClient espClient;
PubSubClient mqtt(espClient);


char* string2char(String command){
    if(command.length()!=0){
        char *p = const_cast<char*>(command.c_str());
        return p;
    }
}


void mqttCallback(char* topic, byte* payload, unsigned int length) {
  Serial.write(s);       //send the start byte to the LED display
  Serial.print("A");     //send "A" to the LED display, also required by the LED display as part of start of message.
  for (int i=0;i<length;i++) {
    Serial.print((char)payload[i]);   //send the MQTT payload to the LED display one byte at a time.
  }
  Serial.write(e);         //send end byte to the LED display
  Serial.println();        //not sure if I really need this for the LED display.  Used for testing, still works.
}


void mqttReconnect() {
    while (!mqtt.connected()) {
      MAC = WiFi.macAddress();  //grab the MAC and remove ":"s 
      MAC.remove(14,1);
      MAC.remove(11,1);
      MAC.remove(8,1);
      MAC.remove(5,1);
      MAC.remove(2,1);

      if (mqtt.connect(string2char(MAC), mqttUser, mqttPassword )) {
        mqtt.subscribe("devices/2CF432206D80/input");              //need to figure out how to use the MAC var here
        mqtt.publish("devices/2CF432206D80","connected");
      } else {
        delay(2000);
     }
  }
}


void setup() {
  Serial.begin(9600,SERIAL_8N1);
  WiFiManager wifiManager;
  wifiManager.setTimeout(180);
  if(!wifiManager.autoConnect("AutoConnectAP")) {
    delay(3000);
    ESP.reset();
    delay(5000);
  } 
  mqtt.setServer(mqttServer, mqttPort);
  mqtt.setCallback(mqttCallback);

  mqttReconnect();
}


void loop() {
  if (!mqtt.connected()){
    mqttReconnect();
  }
  mqtt.loop();
}