Description:
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Feature request. 

Might be nice if the new DateTime::add(), DateTime::sub() and already 
existing DateTime::modify() methods would allow method chaining.





Reproduce code:
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$datetime = new DateTime();
$interval = DateInterval::createFromDateString('2 weeks');

echo $datetime->add($interval)->format('d-m-Y H:i');



Expected result:
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An echo'ed string showing the date of 2 weeks from 'now'.





Actual result:
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Fatal error: Call to a member function format() on a non-object in 
/Users/mathieu/Source/tests/datetime/Modify.php on line 4