php二维数组处理

$arr=array(
    array('amount'=>100,'date'=>'2014-03-29'),
    array('amount'=>120,'date'=>'2014-03-30'),
    array('amount'=>200,'date'=>'2014-03-31')
);

怎么转化成以日期为准，amount为之前amount的和？

$arr=array(
    array('amount'=>100,'date'=>'2014-03-29'),
    array('amount'=>220,'date'=>'2014-03-30'),
    array('amount'=>420,'date'=>'2014-03-31')
);

---

function sd($a,$b) {if($a['date'] == $b['date']) return 0; return $a['date'] >$b['date'] ? 1 : -1;
usort($arr, 'sd');

$amount = 0;
foreach($arr as &$item) {
    $amount += $item['amount'];
    $item['amount'] = $amount;
}

---

<?php

error_reporting(E_ALL);

$arr=array(
    array('amount'=>100,'date'=>'2014-03-29'),
    array('amount'=>200,'date'=>'2014-03-31'),    
    array('amount'=>120,'date'=>'2014-03-30'),

);

$total = 0;
//以日期递增排序
uasort($arr, function($a, $b) {
    return strtotime($a['date']) - strtotime($b['date']);
});

array_walk_recursive($arr, function(&$val, $key) use (&$total) {
    if ($key === 'amount') {
        $total +=  $val;
        $val = $total;
    }
});

print_r($arr);

---

    $originArr=array(
        array('amount'=>100,'date'=>'2014-03-29'),
        array('amount'=>120,'date'=>'2014-03-30'),
        array('amount'=>200,'date'=>'2014-03-31')
    );

    $newArr1 = array();
    $newArr2 = array();
    $tempArr = array();
    $resultArr = array();

    foreach($originArr as $el) {
        $newArr1[$el['date']] = $el['amount'];
    }

    foreach ($newArr1 as $k1 => $v1) {
        foreach ($newArr1 as $k2 => $v2) {
            $v1 += ($k1 > $k2) ? $v2 : 0;
        }
        $newArr2[$k1] = $v1;
     }

     foreach($newArr2 as $key => $value) {
        $tempArr['amount'] = $value;
        $tempArr['date'] = $key;
        $resultArr[] = $tempArr;
     }