ph_mqttsub PowerHome formula function
Description
Allows the specified PowerHome MQTT controller to subscribe to MQTT topics 
Syntax
ph_mqttsub ( as_ctlr, as_topic, ai_qos, ai_type )
Argument Description
as_ctlr String. The ID of the PowerHome MQTT controller
as_topic String. The MQTT topic to subscribe to. Wildcards are allowed
ai_qos Integer. The QOS (Quality of Service) level. Valid values are 0, 1, and 2
ai_type Integer. Specifies how the subscription request is handled within PowerHome. A value of 0 will queue the request with the specified controller. Any other value will cause the the controller to immediately send the request and receive feedback of success/failure
Return value
Integer. Returns 0 if request for subscription was queued successfully when ai_type = 0. Returns 0 if subscription to topic was successful when ai_type <> 0. Returns -1 if the controller ID is not found. Returns -51 if the command is not supported by the controller. Returns -11 if an error occurs during subscription when ai_type <> 0
Usage
Use this function to subscribe to MQTT topics using a currently connected PowerHome MQTT controller. The PowerHome MQTT controller allows you to declare Analog I/O devices with a Publish Topic and a Subscribe Topic and will already be subscribed by the controller. The PH MQTT controller also supports Subscribed topics delcared in the PowerHome Explorer under PowerHome->MQTT->MQTT Subscribe. You would typically use this function to subscribe to topics that are NOT already subscribed to with the Analog I/O section or the MQTT Subscribe section (doing so would be redundant). If you subscribe to a topic using this function that is not already subscribed via Analog I/O or MQTT Subscribe, then the MQTT controller will fire a Generic Controller trigger for the controller on Command 1, Option 1 whenever a published message matching the topic is received. The trigger variables will hold the topic, the message, the QOS, the retain flag, and whether or not the message is a duplicate 
Examples
The following examples demonstrate typical syntax/usage for this function.
• ph_mqttsub("MQTT","home/1stfloor/+/temperature",0,1) - Returns a 0 if the topic subscription was successful. Returns -11 if an error occurred. This example subscribes to a wildcard topic for all temperatures on the 1st floor