Something

                $fns = array('get_results',
                                        'get_row',
                                        'get_value');
                if ( isset( $fetch_fn ) )
                {
                        if ( ! in_array( $fetch_fn, $fns ) )
                        {
                                $fetch_fn = $fns[0];
                        }
                }
                else
                {
                        $fetch_fn = $fns[0];
                }