Usually, you would do something like this:
$post_data = json_encode(array('item' => $post_data));
But, as it seems you want the output to be with “{}“, you better make sure to force json_encode() to encode as object, by passing the JSON_FORCE_OBJECT constant.
$post_data = json_encode(array('item' => $post_data), JSON_FORCE_OBJECT);
“{}” brackets specify an object and “[]” are used for arrays according to JSON specification.