Doctrine\MongoDB\Aggregation\Stage::sort PHP Method

If sorting by multiple fields, the first argument should be an array of field name (key) and order (value) pairs.

See also: http://docs.mongodb.org/manual/reference/operator/aggregation/sort/

public sort ( array \| string $fieldName, integer \| string $order = null ) : Doctrine\MongoDB\Aggregation\Stage\Sort
$fieldName	array \| string	Field name or array of field/order pairs
$order	integer \| string	Field order (if one field is specified)
return	Doctrine\MongoDB\Aggregation\Stage\Sort

    public function sort($fieldName, $order = null)
    {
        return $this->builder->sort($fieldName, $order);
    }

Stage