如何修改数组(array)中的键名(key)？

$a=array();
$a['a']=10;
$a['b']=20;

$a['c']=$a['a'];

unset($a['a']);

foreach($line_center_day_hour_datas as $key=>$value)
{

$line_center_day_hour_datas[$key]['month'] = $value['month'];
$line_center_day_hour_datas[$key]['StatisticTime']  = $value['month'];
unset($line_center_day_hour_datas[$key]['month']);
}