Description:
------------
---
From manual page: http://www.php.net/function.date-diff
---
In some cases date_diff() function shows differenece in days instead of months.

Test script:
---------------
$dateS = new DateTime('2018-03-01');
$dateE = new DateTime('2018-04-01');
print_r(date_diff($dateS, $dateE));

$dateS = new DateTime('2018-02-01');
$dateE = new DateTime('2018-03-01');
print_r(date_diff($dateS, $dateE));

Expected result:
----------------
DateInterval Object
(
    [y] => 0
    [m] => 1
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 0
    [f] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 31
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)
DateInterval Object
(
    [y] => 0
    [m] => 1
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 0
    [f] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)

Actual result:
--------------
DateInterval Object
(
    [y] => 0
    [m] => 1
    [d] => 3//why 3?
    [h] => 0
    [i] => 0
    [s] => 0
    [f] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 31
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)
DateInterval Object
(
    [y] => 0
    [m] => 0//why 0?
    [d] => 28//and why 28?
    [h] => 0
    [i] => 0
    [s] => 0
    [f] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 28
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)