(PECL mongo >=1.0.6)
MongoCursor::fields — Sets the fields for a query
   Fields are specified by "fieldname" : bool.  TRUE 
   indicates that a field should be returned, FALSE indicates that it should
   not be returned.  You can also use 1 and 0 instead of TRUE and FALSE.
  
Thus, to return only the "summary" field, one could say:
<?php
$cursor->fields(array("summary" => true));
?>
To return all fields except the "hidden" field:
<?php
$cursor->fields(array("hidden" => false));
?>
f
     Fields to return (or not return).
Returns this cursor.
Throws MongoCursorException if this cursor has started iterating or a scalar argument is given.