主从结构的JSON要如何分拆

主从结构的JSON要怎么分拆
$a='{"head":{"id":"4","userno":"12345","username":"CDZ1"},"body":[{"orderno":"1","price":"15.00","qty":"1"},{"orderno":"1","price":"9.00","qty":"1"}]}';

这样的一段jsoin, 我要单独取得head 部分的字段值及body 部份的字段值， 比如取 head 部份的 userno,及 body 部份的 price.
------解决思路----------------------
从这样的数组中都不会取出想要的数据，你的老师该打屁股了

array (
  'head' =>   array (
    'id' => '4',
    'userno' => '12345',
    'username' => 'CDZ1',
  ),
  'body' =>   array (
    0 =>     array (
      'orderno' => '1',
      'price' => '15.00',
      'qty' => '1',
    ),
    1 =>     array (
      'orderno' => '1',
      'price' => '9.00',
      'qty' => '1',
    ),
  ),
)

------解决思路----------------------

@header("Content-type: text/html; charset=utf-8");
$a='{"head":{"id":"4","userno":"12345","username":"CDZ1"},"body":[{"orderno":"1","price":"15.00","qty":"1"},{"orderno":"1","price":"9.00","qty":"1"}]}';
$obj=json_decode($a);
echo 'userno:'.$obj->head->userno;
echo '
price:'.$obj->body[0]->price;

/////或：

echo "


";
$ary=json_decode($a,true);
echo 'userno:'.$ary['head']['userno'];
echo '
price:'.$ary['body'][0]['price'];
?>