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To: internals@lists.php.net, "Marco Pivetta" <ocramius@gmail.com>
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Date: Tue, 21 Jul 2015 14:05:54 +0400
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Subject: Re: [PHP-DEV] DateTimeInterface is not a usable interface?
From: inefedor@gmail.com ("Niktia Nefedov")

On Tue, 21 Jul 2015 15:01:06 +0400, Marco Pivetta <ocramius@gmail.com>  
wrote:

> Hello,
>
> I was looking at DateTimeInterface in order to provide my own
> implementation of it, when I hit this:
>
> http://3v4l.org/8GvgO
>
>  > "Fatal error: DateTimeInterface can't be implemented by user classes  
> in"
>
> Is there any actual reason for this kind of limitation?
> Can it be removed before going stable?
>
> I also wasn't able to find any RFC information around it on
> https://wiki.php.net/rfc#php_55 - is there any previous discussion around
> this, or did it just sneak its way in?
>
> Greets,
>
> Marco Pivetta
>
> http://twitter.com/Ocramius
>
> http://ocramius.github.com/

Hey Marco,

The discussion was here https://github.com/php/php-src/pull/512

tl;dr core functions that accept DateTime and DateTimeImmutable  
(DateTimeInterface basically) operate directly on timelib structures hence  
they will break if you pass them a user-defined class. There's a  
workaround though - it's to always call user-defined class's method  
directly from built-in functions, so if you would call (new  
DateTime())->diff(new UserDefinedDateTime()) it would call  
UserDefinedDateTime#diff in the end which would need to implement the  
logic.