为什么我用var_dump就显示全部为NULL，用print_r()就能看见啊

object(mysqli_result)[2]  public 'current_field' => null
 public 'field_count' => null
 public 'lengths' => null
 public 'num_rows' => null
 public 'type' => null

mysqli_result Object ( [current_field] => 0 [field_count] => 3 [lengths] => [num_rows] => 5 [type] => 0 )