DateInterval's properties are not reachable for dynamic inspection (\var_dump(), \print_r(), foreach, etc.)

The returned interval is empty but not NULL. The DateInterval object below has no members when it should have a 1 day difference.

```
$date1 = new DateTime("now");
$date2 = new DateTime("tomorrow");
$diff = $date1->diff($date2);
print_r($diff)
```