Description:
------------
DateTime validation is incomplete. According to DateTime::__construct 
documentation this is not a bug:

// Non-existent values roll over.
$date = new DateTime('2000-02-30');
echo $date->format('Y-m-d H:i:sP') . "\n";

I disagree. There should be at least some way to require a proper validation (such 
as checkdate() function provides) for input dates. If you think this is incorrect 
behavior and rolling over is the desired one, then please implement the rolling so 
that one can for example give "2000-02-37" as the argument. Now DateTime checks 
the day of the month like this: 0 <= x <= 31.

Test script:
---------------
<?php
$date = new DateTime('2011-02-31');
print $date->format('Y-m-d');

$date = new DateTime('2011-02-32');
print $date->format('Y-m-d');
?>


Expected result:
----------------
Both DateTime constructors should throw an Exception describing that the date is 
invalid.

Actual result:
--------------
Now the constructor with '2011-02-32' as its parameter is the only one throwing 
Exception about invalid date.