Query with last records grouped by 1 field

I'm using PHP with the MongoDb DRIVER.

I can use simple query using sorts , selecting a record by id, but I want to make a query that takes the last records of the collection, grouped by idHostName .

In the document I have the AliveBees Collection with the fields (_id,_idHostName,CollectDates)

In MongoDB the query works:

db.AliveBees.aggregate(
   [
     { $sort: { CollectDate: 1, _idHostName: 1 } },
     {
       $group:
         {
           _id: "$_idHostName",
             "_idHostName": {"$last":"$_idHostName"},
             lastCollectDate: { $last: "$CollectDate" }
         }         
     }
   ]
)

But in PHP I can not mount it to return this data to me.