<?php
$foo = array(
    '21-8-2009' => 'foo',
    '22-8-2008' => 'bar',
    '30-9-2011' => 'baz',
);
uksort($foo, function($a, $b) {
    $time_a = strtotime($a);
    $time_b = strtotime($b);
    if ($time_a == $time_b) {
        return 0;
    }
    if ($time_a > $time_b) {
        return 1;
    }
    return -1;
});
print_r($foo);