Description:
------------
---
From manual page: http://www.php.net/datetime.sub#refsect1-datetime.sub-
description
---

When you have a datetime object you can modify the date adding or subtracting 
month. If you do that, the result is always unpredictable, because the mktime, 
the sub, or the modify function, always subtract the amount of days of the 
previous month. I'll make an example:
If I do
<?php
$date = new DateTime('2001-03-31');
$interval = new DateInterval('P1M');

$date->sub($interval);
echo $date->format('Y-m-d');
?>
I obtain 2001-03-03 as result, but I've expected to have 2001-02-28, because the 
previous month is february, not 28 days first.
We have the same problem if we add some months to our starting date:

<?php
$date = new DateTime('2001-03-31');
$interval = new DateInterval('P1M');

$date->add($interval);
echo $date->format('Y-m-d');
?>
I obtain 2001-05-01 as result, but also in this case I've aspected a different 
result, i think the right value is 2001-04-30, because you have to add the days 
of the next month to obtain the right result, not the days of current month.
.
The problem is also the communication with mySql, because mySql calculate the 
right result, so the comparison between two date it's impossible

Test script:
---------------
<?php
$date = new DateTime('2001-03-31');
$interval = new DateInterval('P1M');

$date->sub($interval);
echo $date->format('Y-m-d');
?>

Expected result:
----------------
2001-02-28

Actual result:
--------------
2001-03-03