<?php

$arr = Array(
  11 => Array (
    4 => 'apr11timetable.php', 
    8 => 'aug11timetable.php', 
    6 => 'jun11timetable.php', 
    11 => 'nov11timetable.php', 
    10 => 'oct11timetable.php', 
  ),

  12 => Array (
    4 => 'apr12timetable.php', 
    8 => 'aug12timetable.php', 
    2 => 'feb12timetable.php', 
    6 => 'jun12timetable.php', 
    10 => 'oct12timetable.php', 
  ),
);

print_r($arr);

foreach ($arr as &$ar) { arsort($ar); }

print_r($arr);

Array
(
    [11] => Array
        (
            [4] => apr11timetable.php
            [8] => aug11timetable.php
            [6] => jun11timetable.php
            [11] => nov11timetable.php
            [10] => oct11timetable.php
        )

    [12] => Array
        (
            [4] => apr12timetable.php
            [8] => aug12timetable.php
            [2] => feb12timetable.php
            [6] => jun12timetable.php
            [10] => oct12timetable.php
        )

)
Array
(
    [11] => Array
        (
            [10] => oct11timetable.php
            [11] => nov11timetable.php
            [6] => jun11timetable.php
            [8] => aug11timetable.php
            [4] => apr11timetable.php
        )

    [12] => Array
        (
            [10] => oct12timetable.php
            [6] => jun12timetable.php
            [2] => feb12timetable.php
            [8] => aug12timetable.php
            [4] => apr12timetable.php
        )

)