Description:
------------
Today is 2015-02-02 (02.February)

I want first and last day ob last week:

$date = new DateTime('first day of last week');
echo $date->format('Y-m-d');
// ---
$date = new DateTime('last day of last week');
echo $date->format('Y-m-d');

result is (WRONG!):

2015-02-01
2015-02-28

MUST BE:

2015-01-26
2015-02-01

This working fine with "month" instead "week":

DateTime('first day of last month');
DateTime('last day of last month');

result is:

2015-01-01
2015-01-31

Test script:
---------------
$date = new DateTime('first day of last week');
echo $date->format('Y-m-d');

$date = new DateTime('last day of last week');
echo $date->format('Y-m-d');


Expected result:
----------------
2015-01-26
2015-02-01

Actual result:
--------------
2015-02-01
2015-02-28