The method Collection::bulkWrite() takes a list of operations as first argument. It is an array of array of array as shown by the example: https://github.com/mongodb/mongo-php-library/blob/master/examples/bulk.php

In order to provide a better typing, the method should accept the PHPC model class: MongoDB\Driver\BulkWrite.

Advantages :

• DX: developers are guided by the method signatures
• Performance: Less validation and transformation code to execute in 
  MongoDB\Operation\BulkWrite
• DX: Leverage the feature of the library client, instead of calling the MongoDB\Driver\Manager directly