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"}