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Date: Fri, 16 Jan 2015 16:22:05 +0000
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Subject: Re: [PHP-DEV] Fixing strange foreach behavior.
From: rowan.collins@gmail.com (Rowan Collins)

Yasuo Ohgaki wrote on 16/01/2015 08:40:
> Hi all,
>
> Take a look at
>
> http://3v4l.org/HbVnd
>
> foreach should not affect internal(zval) array position, but it does.
> I'm not sure why foreach works this way, but HHVM behavior is
> reasonable. IMHO.
>
> PHP 7 would be perfect opportunity fix this behavior.
> Any comments?

I seem to remember somebody mentioning that the pointer is reused in 
some situations and not others. I think the reason is efficiency - we 
have a pointer associated with each array, which is rarely used, so why 
not borrow it for foreach? (Answer: because it causes the oddity you 
just spotted. :P)

For instance, if you put a reset() in the middle of the foreach loop, it 
doesn't interrupt the progress of the loop, but does cause it to be 
tracked separately and output "Apple" after the loop finishes: 
http://3v4l.org/KTCrI

There are other weird cases, like nesting two foreach loops over the 
same variable:
- with a true array, the loops are independent (both loops see the full 
set of values): http://3v4l.org/lJTQG
- with a rewindable Traversable, such as SimpleXMLElement, they interact 
(the inner loop sees all values, but the outer loop doesn't continue 
after the inner one completes): http://3v4l.org/fs2Jv
- with a non-rewindable Traversable, such as a Generator, PHP tries to 
rewind and gives a fatal error, while HHVM simply uses up the last 
values of the generator on the inner loop (possibly a bug?): 
http://3v4l.org/BYOmQ

My conclusion: don't try and nest foreach or any related constructs 
using the same variable, because the behaviour is not guaranteed to be 
in any way sane.

Regards,
-- 
Rowan Collins
[IMSoP]