Description:
------------
http://www.php.net/manual/en/datetime.sub.php

documentation says:
DateTime::sub —  Subtracts an amount of days, months, years, hours, minutes and seconds from a DateTime object 

Return Values
Returns the modified DateTime object or FALSE on failure. 

this is fine, but it is not clear that "sub" or "add" modify value on called instance, and then - really dont know why - returns that instance (again).


Test script:
---------------
$d1 = new DateTime();
$d2 = $d1->sub(new DateInterval('P1W'));

var_dump($d1);
var_dump($d2);

var_dump($d1 === $d2);

Expected result:
----------------
object(DateTime)[1]
  public 'date' => string '2010-06-12 22:01:55' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/Wdoltown' (length=13)

object(DateTime)[2]
  public 'date' => string '2010-06-05 22:01:55' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/Wdoltown' (length=13)

boolean false


Actual result:
--------------
object(DateTime)[1]
  public 'date' => string '2010-06-05 22:01:55' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/Wdoltown' (length=13)

object(DateTime)[1]
  public 'date' => string '2010-06-05 22:01:55' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/Wdoltown' (length=13)

boolean true