Description:
------------
When you subclass DateTime, PHP insists on there being no type hint present for `add()` and `sub()`. If you type-hint them to `DateInterval`, php throws a warning about the signature not matching the parent, even though the manual (and reality agrees) says that the argument is a DateTimeInterval.

While I would personally love for this warning to be turnoaffable or at least not be a warning but a notice, it certainy shouldn't warn for correct usage.

As it stands now, code that was using type hints issues warnings until the type hints are removed, even though DateTime::add() also issues a warning if you pass anything but a DateInterval as an argument.

Test script:
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<?php

class Foo extends DateTime {
    function add(DateInterval $interval){ parent::add($interval); }
}


Expected result:
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no output. No warning

Actual result:
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PHP Warning:  Declaration of Foo::add() should be compatible with DateTime::add($interval) in foo.php on line 5