<?php
    //set date array to list of dates in y-d-m format
    $date=array('2009-03','2010-01','2009-02');
    for ($i=0;isset($date[$i]);$i++) {
    $datess=explode('-',$date[$i]);

    $dates[$i]=$datess[0].'-'.$datess[1];
    unset($datess);
    }
    unset($date);
    asort($dates);
    for ($i=0;isset($dates[$i]);$i++) {
    $datess=explode('-',$dates[$i]);
    $date[$i]=$datess[1].'-'.$datess[0];
    unset($datess);
    }
    print_r($date);
?>

Array
(
    [0] => 03-2009
    [1] => 01-2010
    [2] => 02-2009
)