Programmer - which method is more efficient: set a class property; or return the result?

Programmer Question

Please consider this example. (PHP)

class Database{  
    private $result;  
    private $conn;

    function query($sql){  
        $result = $this->conn->query($sql);  
        // Set the Database::$result ?? Or return the value and avoid setting property?  
        return $result;
        // $this->result = $result;  
    }  
}

what are the advantages of both these methods? Where are they applicable?