Description:
------------
I used this function as below but i have problem in February  month
if 'user_invoice_date' equal 2012-01-30 or 2012-01-31
the 'new_invoice_date' equal 2012-03-01
$new_invoice_date=date("Y-m-01 H:i:s",strtotime($user_invoice_date."+1 month"));

Test script:
---------------
normal 
$user_invoice_date="2011-12-31";
$new_invoice_date=date("Y-m-01 H:i:s",strtotime($user_invoice_date."+1 month"));
////////////////
Error
$user_invoice_date="2012-01-31";
$new_invoice_date=date("Y-m-01 H:i:s",strtotime($user_invoice_date."+1 month"));
echo $new_invoice_date;

Expected result:
----------------
$user_invoice_date="2012-01-31";
$new_invoice_date=date("Y-m-01 H:i:s",strtotime($user_invoice_date."+1 month"));

$new_invoice_date=2012-02-01

2012-02-01

Actual result:
--------------
$new_invoice_date=2012-03-01

2012-03-01