<?php

 $allarticles = Array (Array ("id" => 24,
                                     "article_id" => 74,
                                     "created" => "2011-01-01 20:48:48" ),
                       Array ( "id" => 39,
                                      "article_id" => 94,
                                      "created" => "2011-02-21 21:06:44" ) 
                      );

function csort($a, $b) {
    return strcmp($b['created'], $a['created']);
}

usort($allarticles, 'csort');

print_r($allarticles);

Array
(
    [0] => Array
        (
            [id] => 39
            [article_id] => 94
            [created] => 2011-02-21 21:06:44
        )

    [1] => Array
        (
            [id] => 24
            [article_id] => 74
            [created] => 2011-01-01 20:48:48
        )

)