insert json object to mongodb array field from php

I am writing a php script to insert a json object to an array field of a mongodb document . Specifically in the mongodb doc I want to push an object in the format of {'user': usr , 'commented':comment} to the field 'comments':[] in a mongodb doc . The problem is how to create this json object using php as this is my first time trying this .

This is what I have so far :

//find the correct item 
     $review  = array(
        "user"=>$usr,
        "comment"=>$comm,
        "rated"=>$rated
      ); 
    
    $productCollection->updateOne(array('id'=>$itmID),
       array('$push'=>array('comments'=>$review))
      );