php的json_encode函数问题

php的json_encode函数问题：

$ary = [];
$ary[0] = 'a';
$ary[1] = 'b';
echo json_encode($ary) . '<br>';
echo json_encode($ary, JSON_FORCE_OBJECT);

结果：

["a","b"]
{"0":"a","1":"b"}