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Question about `global` variable declaration

php.internals

Kosit Supanyo

6 years ago

Hi Internals

I'm working on my new proposals and I've found weirdness of global variable
declaration in zend_language_parser.y.

global_var:
simple_variable
{ $$ = zend_ast_create(ZEND_AST_GLOBAL,
zend_ast_create(ZEND_AST_VAR, $1)); }
;

Above grammer allows something like this...

global $$x;
global $$$y;
global $$$$z;
global ${'abc'};
global $$$$$$$$$${random_int(0, PHP_INT_MAX)};

What's the propose of allowing this? And is there anyone out there knowing
and using it? If not, should this be changed to allow only T_VARIABLE to
make it consistent with `static` variable declaration?

Regards

Dan Ackroyd

6 years ago

Hi Kosit,

On Sat, 21 Sep 2019 at 10:56, Kosit Supanyo <webdevxp.com@gmail.com> wrote:

>
> global $$x;
> global $$$y;
> global $$$$z;
> global ${'abc'};
> global $$$$$$$$$${random_int(0, PHP_INT_MAX)};

I have found the form `global ${'abc'};` useful in the past to work
around the limits on what PHP can parse as variable names, to allow
naming global settings to match how I like to write them in config
files e.g. 'image.upload.retry_attempts'. I would be very surprised if
there was no codebase in existence that didn't use the other forms,
for better or worse.

> If not, should this be changed to ...

For changes like this, the conversation almost certainly needs to
start by stating a clear problem something is causing, and then lead
on to why doing a suggested solution is the right thing to do.

Although this might be a small inconsistency in the language, by
itself, I can't see it causing much harm, except maybe for performance
optimisation as trying to analyze what variables are being used in the
code is hard.

cheers
Dan
Ack

Nikita Popov

6 years ago

On Sat, Sep 21, 2019 at 11:56 AM Kosit Supanyo <webdevxp.com@gmail.com>
wrote:

> Hi Internals
>
> I'm working on my new proposals and I've found weirdness of global variable
> declaration in zend_language_parser.y.
>
> global_var:
>     simple_variable
>         { $$ = zend_ast_create(ZEND_AST_GLOBAL,
> zend_ast_create(ZEND_AST_VAR, $1)); }
> ;
>
> Above grammer allows something like this...
>
> global $$x;
> global $$$y;
> global $$$$z;
> global ${'abc'};
> global $$$$$$$$$${random_int(0, PHP_INT_MAX)};
>
> What's the propose of allowing this? And is there anyone out there knowing
> and using it? If not, should this be changed to allow only T_VARIABLE to
> make it consistent with `static` variable declaration?
>
> Regards
>

Some grep results:

sources/adodb/adodb-php/session/adodb-session.php
676: global $$var;
697: global $$var;

sources/adodb/adodb-php/session/adodb-session2.php
719: global $$var;
740: global $$var;

sources/adodb/adodb-php/session/old/adodb-cryptsession.php
189: global $$var;

sources/adodb/adodb-php/session/old/adodb-session.php
300: global $$var;

sources/adodb/adodb-php/session/old/adodb-session-clob.php
269: global $$var;

sources/wp-cli/wp-cli/php/WP_CLI/Runner.php
1177: global ${$key};

sources/apache/log4php/src/main/php/pattern/LoggerPatternConverterSuperglobal.php
71: global ${$this->name};

We could deprecate this in favor of $GLOBALS, though as Dan said, the
motivation is not quite clear right now.

Nikita

Kosit Supanyo

6 years ago

Hi Dan and Internals

Sorry  Dan, I forgot to include @Internals in previous reply so let me
resend this again.

Thank you for your reply. I see, but in that case it can be done with
`$GLOBALS['abc']` right? So I don't see any benefits of allowing those
forms, they're just another inconsistency that should not exist from the
beginning. Yes, it does no harms but if nobody is really using it at all,
is it good to remove this inconsistency? Or to make it really useful, why
not just allow assignment like:

global ${'abc'} = $someValue;

Just my 2 cents.

Regards

On Sat, Sep 21, 2019 at 5:52 PM Nikita Popov <nikita.ppv@gmail.com> wrote:

Nikita Popov

6 years ago

On Sat, Sep 21, 2019 at 12:58 PM Kosit Supanyo <webdevxp.com@gmail.com>
wrote:

> Hi Dan and Internals
>
> Sorry  Dan, I forgot to include @Internals in previous reply so let me
> resend this again.
>
> Thank you for your reply. I see, but in that case it can be done with
> `$GLOBALS['abc']` right? So I don't see any benefits of allowing those
> forms, they're just another inconsistency that should not exist from the
> beginning. Yes, it does no harms but if nobody is really using it at all,
> is it good to remove this inconsistency? Or to make it really useful, why
> not just allow assignment like:
>
> global ${'abc'} = $someValue;
>
> Just my 2 cents.
>
> Regards
>

global $x; is a shorthand for $x =& $GLOBALS['x'];
global ${$x}; is a shorthand for ${$x} =& $GLOBALS[$x];

I don't think this syntax is inconsistent so long as PHP supports the
syntax ${$x} for variables in general.

The idea of a "global ${'abc'} = $someValue;" syntax doesn't seem to have a
relation to the ${} form in particular. A simple "global $var = $someValue"
is currently not supported either. That's because "global $var" simply
imports a global variable into the local scope. You would instead write
this as "global $var; $var = $someValue;".

Nikita

Kosit Supanyo

6 years ago

I understand your point but inconsistency in my sense is syntactical By
comparing to other declaration syntax like `var`, `static`, 'public` an
others. They allow only T_VARIABLE but `global` is different. And there's
another way to archive the same goal through `$GLOBALS`, that's why I see
it as inconsistency.

P.S. I wrote this not because I want to convince you to make any change but
just to explain my point.

Cheers

On Sat, Sep 21, 2019 at 6:24 PM Nikita Popov <nikita.ppv@gmail.com> wrote:

Zeev Suraski

6 years ago

On Sat, Sep 21, 2019 at 3:09 PM Kosit Supanyo <webdevxp.com@gmail.com>
wrote:

> I understand your point but inconsistency in my sense is syntactical By
> comparing to other declaration syntax like `var`, `static`, 'public` an
> others. They allow only T_VARIABLE but `global` is different. And there's
> another way to archive the same goal through `$GLOBALS`, that's why I see
> it as inconsistency.
>

Even though we're dealing with archeology here - this really isn't an
inconsistency but intended behavior.
Unlike var, public, static and others - 'global' is not a declaration of
class structure, but a way to access global variables.  In the same way it
sometimes makes sense to access variables indirectly ($$foo), it may
sometimes make sense to access global variables indirectly (global $$foo).
It allows for both creation and access of global variables.  It doesn't
sound very useful or practical to provide that ability for the creation of
dynamic class properties, which is why it's not available for
var/public/static/etc.

Zeev

Kosit Supanyo

6 years ago

>
> Unlike var, public, static and others - 'global' is not a declaration of
> class structure, but a way to access global variables.

I know it is not and I think almost everyone knows that. As I said, I came
up with this by comparing to other declarations syntactically not
functionally, it is inconsistent in that way. I don't know how many PHP
developers out there know `global` can take variable variables but I my
guess is, very very little because there's no information about this
behavior of `global` in PHP manual
https://www.php.net/manual/en/language.variables.scope.php and I'd never
seen those forms of `global` in any projects/articles before.

Cheers

On Sat, Sep 21, 2019 at 9:26 PM Zeev Suraski <zeev@php.net> wrote:

Zeev Suraski

6 years ago

On Sat, Sep 21, 2019 at 6:18 PM Kosit Supanyo <webdevxp.com@gmail.com>
wrote:

> Unlike var, public, static and others - 'global' is not a declaration of
>> class structure, but a way to access global variables.
>
>
> I know it is not and I think almost everyone knows that. As I said, I came
> up with this by comparing to other declarations syntactically not
> functionally, it is inconsistent in that way. I don't know how many PHP
> developers out there know `global` can take variable variables but I my
> guess is, very very little because there's no information about this
> behavior of `global` in PHP manual
> https://www.php.net/manual/en/language.variables.scope.php and I'd never
> seen those forms of `global` in any projects/articles before.
>

I think you're raising two separate points:

1. The global statement is inconsistent with other similar statements.
2. It's not widely used and we should consider deprecating this behavior.

Re: #1 - even though it may look similar, the global statement is
fundamentally different from public/private/static and friends.  It's not
really a declaration - it's like Nikita said is a simple equivalent for
writing $foo = &$GLOBALS['foo'] - and as such, is a runtime statement as
opposed to a compile-time declaration like the others.  So despite the
superficial similarities, the differences between the two justify different
behavior/allowed syntax.

Re: #2 - the fact it's not documented is a fair point - but I'm not sure
that in itself is cause for anything other than perhaps adding a quick note
in the docs.  People familiar with PHP's variable-variables functionality (
https://www.php.net/manual/en/language.variables.variable.php) will not
find this behavior as inconsistent or otherwise surprising, I think.  It
does appear to be mentioned in some places (e.g. the O'Reilly PHP Cookbook,
https://www.oreilly.com/library/view/php-cookbook/1565926811/ch06s12.html).

I doubt there'll be a big impact if we were to deprecate this behavior - it
doesn't appear very mainstream (and there's a simple workaround available
through $GLOBALS) - but I don't see any gain to be had either.

Zeev