Despite the line above that says that the $step value should be "given as a positive number," the range() function will in fact correctly handle reversed (decrementing) ranges. For example:

<?php print_r( range( 24, 20 ) ); ?>
Array
(
    [0] => 24
    [1] => 23
    [2] => 22
    [3] => 21
    [4] => 20
)

<?php print_r( range( 20, 11, -3 ) ); ?>
Array
(
    [0] => 20
    [1] => 17
    [2] => 14
    [3] => 11
)

It will actually ignore the sign of the $step argument, and determine whether to increment or decrement based purely on whether $start > $end or $end > $start. For example:

<?php print_r( range( 20, 11, 3 ) ); ?>
Array
(
    [0] => 20
    [1] => 17
    [2] => 14
    [3] => 11
)

<?php print_r( range( 11, 20, -3 ) ); ?>
Array
(
    [0] => 11
    [1] => 14
    [2] => 17
    [3] => 20
)