This is an old post but probably somebody can use this info:

void mqtt_callback(char* topic, byte* payload, unsigned int length) { String topicStr = topic; String recv_payload = String(( char *) payload);

Serial.println( "mqtt_callback - message arrived - topic [" + topicStr + "] payload [" + recv_payload + "]" ); }

This is an old post but probably somebody can use this info:

void mqtt_callback(char* topic, byte* payload, unsigned int length) {
  String topicStr = topic; 
  String recv_payload = String(( char *) payload);

  Serial.println( "mqtt_callback - message arrived - topic [" + topicStr + 
                  "] payload [" + recv_payload + "]" );
}