Here is a small and very fast object to handle sorting of multidimentional arrays by a key.
<?php
/**
* Handles multidimentional array sorting by a key (not recursive)
*
* @author Oliwier Ptak <aleczapka at gmx dot net>
*/
class array_sorter
{
    var $skey = false;
    var $sarray = false;
    var $sasc = true;

    /**
    * Constructor
    *
    * @access public
    * @param mixed $array array to sort
    * @param string $key array key to sort by
    * @param boolean $asc sort order (ascending or descending)
    */
    function array_sorter(&$array, $key, $asc=true)
    {
        $this->sarray = $array;
        $this->skey = $key;
        $this->sasc = $asc;
    }

    /**
    * Sort method
    *
    * @access public
    * @param boolean $remap if true reindex the array to rewrite indexes
    */
    function sortit($remap=true)
    {
        $array = &$this->sarray;
        uksort($array, array($this, "_as_cmp"));
        if ($remap)
        {
            $tmp = array();
            while (list($id, $data) = each($array))
                $tmp[] = $data;
            return $tmp;
        }
        return $array;
    }

    /**
    * Custom sort function
    *
    * @access private
    * @param mixed $a an array entry
    * @param mixed $b an array entry
    */
    function _as_cmp($a, $b)
    {
        //since uksort will pass here only indexes get real values from our array
        if (!is_array($a) && !is_array($b))
        {
            $a = $this->sarray[$a][$this->skey];
            $b = $this->sarray[$b][$this->skey];
        }

        //if string - use string comparision
        if (!ctype_digit($a) && !ctype_digit($b))
        {
            if ($this->sasc)
                return strcasecmp($a, $b);
            else 
                return strcasecmp($b, $a);
        }
        else
        {
            if (intval($a) == intval($b)) 
                return 0;

            if ($this->sasc)
                return (intval($a) > intval($b)) ? -1 : 1;
            else
                return (intval($a) > intval($b)) ? 1 : -1;
        }
    }

}//end of class
?>

Sample $input_array:

Array
(
    [0] => Array
        (
            [id] => 961
            [uid] => 29
            [gid] => 12
            [parent_id] => 147
            [created] => 20041206105350
            [modified] => 20041206110702
        )

    [1] => Array
        (
            [id] => 41
            [uid] => 29
            [gid] => 12
            [parent_id] => 153
            [created] => 20041025154009
            [modified] => 20041206105532
        )

    [2] => Array
        (
            [id] => 703
            [uid] => 29
            [gid] => 12
            [parent_id] => 419
            [created] => 20041025154132
            [modified] => 20041027150259
        )

Example of usage:
<?php
    function multi_sort(&$array, $key, $asc=true)
    {
        $sorter = new array_sorter($array, $key, $asc);
        return $sorter->sortit();
    }
    //sort by parent_id in descending order
    $my_array = multi_sort($input_array, "parent_id", false);
?>

The result array will be:
Array
(

    [0] => Array
        (
            [id] => 703
            [uid] => 29
            [gid] => 12
            [parent_id] => 419
            [created] => 20041025154132
            [modified] => 20041027150259
        )

    [1] => Array
        (
            [id] => 41
            [uid] => 29
            [gid] => 12
            [parent_id] => 153
            [created] => 20041025154009
            [modified] => 20041206105532
        )

    [2] => Array
        (
            [id] => 961
            [uid] => 29
            [gid] => 12
            [parent_id] => 147
            [created] => 20041206105350
            [modified] => 20041206110702
        )