$arr=array(array('amount'=>100,'date'=>'2014-03-29'),array('amount'=>120,'date'=>'2014-03-30'),array('amount'=>200,'date'=>'2014-03-31'));怎么轉化成以日期為準，amount為之前amount的和？$arr=array(array('amount'=>100,'date'=>'2014-03-29'),array('amount'=>220,'date'=>'2014-03-30'),array('amount'=>420,'date'=>'2014-03-31'));