Refactor mid() function

[simobern]

function mid($id = null) {
  return is_a($id, 'MongoDB\BSON\ObjectId') ? $id : new MongoDB\BSON\ObjectId($id);
}

[iamdaniele]

Would class_alias be a better alternative?

class_alias('MongoDB\BSON\ObjectId', 'MongoId');

This way, we can:

• Make sure the new BaseType will work with classes in the global namespace (un-namespaced)
• Preserve compatibility with the old code using MongoId pretty much universally