Looking at the life of messageIDs, you create initialise `$lastMessageID` to -1 in the constructor, then set `$messageID` to be it's value as you increment it in the send method, line 109.

It's used in sendMessages if you are sending a lot of messages so that it knows where to start again from if there is an error. It seems to have no purpose outside the object, so there is no point returning it, especially if there is nothing similar to return for other message types.