Once the function is implemented, the following TODO from phongo_writeresult_init() should be completed:

bson_to_zval() requires an array or document, so we'll convert the entire upsert document to a zval. Ideally, we would recurse within the document, use bson_iter_value() to access the "_id" field, and then convert that alone. The "index" field could be read directly as an integer and used with add_index_zval().