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Date: Tue, 09 Jun 2015 15:09:31 +0200
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Subject: Re: [PHP-DEV] Array dereferencing of scalars
From: cmbecker69@gmx.de (Christoph Becker)

Rowan Collins wrote:

> Yasuo Ohgaki wrote on 09/06/2015 11:44:
>>   $v = NULL;
>>   $v[0][1][2][3][4][5][6][7][8][9]; // NULL
>>
>> this code is semantically wrong and I would like to have error/exception
>> for such
>> erroneous codes.
> 
> PHP considers an uninitialised variable to have the value NULL, and a
> NULL value to be coercable to any type, so this breaks down (logically,
> not necessarily literally) as follows:
> 
> - coerce $v to array()
> - instantiate $v[0] as NULL
> - coerce $v[0] to array()
> - instantiate $v[0][1] as NULL
> - and so on...
> 
> Raising a notice whenever the type is coerced would be inconsistent with
> other coercions (e.g. $foo = null; echo $foo + 1;). Raising a notice
> whenever the coerced array is actually accessed would result in a notice
> for every dimension, which would be very noisy.
> 
> Ideally, it would give a single notice, as in the below, but I'm not
> sure how that would be implemented:
> 
>  $v = array();
>  var_dump($v[0][1][2][3][4][5][6][7][8][9]);
>  // Notice: Undefined index: 0
>  // NULL
> 
> Note that this is all rather different from the original case, which was
> about values which *cannot be coerced to array*.

I wonder where these coercion rules are described.  The manual has a
section about "Converting to array"[1] which actually describes casting,
and is obviously not what is happening when the subscript operator (as
the langspec calls it[2]) is applied to non-null scalars.

The section "Accessing array elements with square bracket syntax"[3]
doesn't describe the behavior of accessing elements of non-arrays at all.

The language specification specifies as constraint:

| If subscript-expression is used in a non-lvalue context, the element
| being designated must exist.

Finally, it is not even clear why a value should be coerced to array
when the subscript operator is applied – it might be coerced to string
as well.

[1]
<http://php.net/manual/en/language.types.array.php#language.types.array.casting>
[2]
<https://github.com/php/php-langspec/blob/master/spec/10-expressions.md#subscript-operator>
[3]
<http://php.net/manual/en/language.types.array.php#language.types.array.syntax.accessing>

-- 
Christoph M. Becker