How does the PHP MongoDB Driver's Cursor buffer a result set?

When queries are made to mongodb, how does it's cursor deal with the result set in memory? Does the cursor retrieve all documents which match the query, at once? or does it retrieve 1 document at a time? or are they buffered? or is there a different solution I don't know about?

If it's a buffered solution, how are they stored on the server/client? How much data does the client keep locally?