PHP 对二维数组排序

现在有一个二维数组

$arr = array(
    array('date'=>'2011-04-21','wu'=>'test'),
    array('date'=>'2014-04-21','wu'=>'good'),
    array('date'=>'2011-12-22','wu'=>'great'),
    array('date'=>'2012-05-21','wu'=>'hahah'),
    array('date'=>'2013-08-13','wu'=>'test'),
    array('date'=>'2011-04-25','wu'=>'yes'),
    array('date'=>'2014-04-14','wu'=>'no'),
);

我想将它按照里面date日期来重新排序，得到这样的新数组：

$arr2 = array(
    array('date'=>'2011-04-21','wu'=>'test'),
    array('date'=>'2011-04-25','wu'=>'yes'),
    array('date'=>'2011-12-22','wu'=>'great'),
    array('date'=>'2012-05-21','wu'=>'hahah'),
    array('date'=>'2013-08-13','wu'=>'test'),
    array('date'=>'2014-04-14','wu'=>'no'),
    array('date'=>'2014-04-21','wu'=>'good'),
);

请问应该怎样做呢？

---

直接上代码

$tmp = array();

foreach ($arr as $a) {
    $tmp[$a['date'] . '_' . $a['wu'] . '_' . rand(1000000, 9999999)] = $a;
}

asort($tmp);

$arr = array_values($tmp);

---

使用 array_multisort
参考：http://cn2.php.net/array_multisort

php$sort = array(  
    'direction' => 'SORT_ASC', //排序顺序标志 SORT_DESC 降序；SORT_ASC 升序  
    'field'     => 'date',       //排序字段  
);
$arrSortTmp = array();  
foreach($arr AS $uniqid => $row){  
    foreach($row AS $key=>$value){  
        $arrSortTmp[$key][$uniqid] = $value;  
    }  
}
array_multisort($arrSortTmp[$sort["field"]],constant($sort["direction"]), $arr);

var_dump($arr);

---

$arr = array(
array('date'=>'2011-04-21','wu'=>'test'),
array('date'=>'2014-04-21','wu'=>'good'),
array('date'=>'2011-12-22','wu'=>'great'),
array('date'=>'2012-05-21','wu'=>'hahah'),
array('date'=>'2013-08-13','wu'=>'test'),
array('date'=>'2011-04-25','wu'=>'yes'),
array('date'=>'2014-04-14','wu'=>'no'),
);

array_multisort($arr, SORT_ASC);

print_r($arr);