Description:
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Calling $dt->modify("Monday this week") for all dates 13–19 May 2012 (Sun–Sat) 
result in the same date (2012-05-14), suggesting that "this week" runs Sun–Sat. 
However, calling $dt->modify("Sunday this week") gives the "next" Sunday (2012-
05-20) for the same range.

On a related note, the documentation on relative date formats is lacking on the 
behaviour of this and strtotime when used with locales; one might expect the 
start of the week to be interpreted from LC_TIME.

Test script:
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<?php

$dt = new DateTime("2012-05-13");
$dt->modify("Sunday this week");
var_dump($dt->format('r'));

Expected result:
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string(31) "Sun, 13 May 2012 00:00:00 +0000"

Actual result:
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string(31) "Sun, 20 May 2012 00:00:00 +0000"