public static function sort_months($base_directory) {
        $month_year_array = array();
        $old_year = "";
        $years_array =array();
        $sorted_array = array();

        for($index = 0; $index < count($base_directory); $index++) {
            // april_2009
            $current_directory = $base_directory[$index];
            // ("2009" => "april")
            $complete_month_of_year_array = array();
            // 2009
            $new_year = substr($current_directory, strpos($current_directory, "_") + 1);

            if ($old_year != $new_year) {
                // "april"
                $month_array = array();

                foreach ($base_directory as $directory) {
                    if(substr($directory, strpos($directory, "_") + 1) == $new_year) {
                        array_push($month_array, $directory);
                    }
                }

                if (count($month_array) > 0) {
                    $complete_month_of_year_array = array($new_year => $month_array);
                    array_push($month_year_array, array($new_year => $month_array));
                    array_push($years_array, $new_year);
                }
            }

            $old_year = $new_year;
        }

        sort($years_array);
        print_r($month_year_array);

        foreach ($years_array as $year) {
            $current =  $month_year_array[$year];
            foreach ($current as $d) {
                array_push($sorted_array, $d);
            }
        }

        print_r($sorted_array);
    }

output

Array ( [0] => Array ( [2010] => Array ( [0] => april_2010 [1] => march_2010 ) ) [1] => Array ( [2009] => Array ( [0] => januari_2009 [1] => april_2009 ) ) )

prefered Array ( [2010] => Array ( [0] => april_2010 [1] => march_2010 )  [2009] => Array ( [0] => januari_2009 [1] => april_2009 )  )