Not a PHP expert (yet ) so forgive me if I am misunderstanding your question.
You have a collection of documents that represent posts in your system and each one of them contains an array of comments. You are trying to add a new comment to an already existing post. You are using the right method but it needs two arguments: one to indicate the document you want to update, and another with the expected update.`
I cannot test this code at the moment, but with the mercy of the syntax gods, it should at least help you understand what was the problem with your code.
As an aside, I would suggest not using an Id for each of the comments, since they are embedded objects accessed via the main document.
Hope this helps.
Jorge