Description:
------------
When trying to set or get any pre-defined property other then y/m/d/h/i/s in a class extended from built-in DateInterval, the error tells you that the property is not known.

Also tried with 
PHP 5.3.4-dev (cli) (built: Aug 30 2010 13:48:30) 

Test script:
---------------
<?php
class di extends DateInterval {
    public $unit = 1;
}

$I = new di('P10D');
echo $I->unit;


Expected result:
----------------
1

Actual result:
--------------
Fatal error: main(): Unknown property (unit) in /var/www/test/inter.php on line 7