${} is deprecated without documenting new behaviour in PHP 9.0

[Flimm]

Description

According to this accepted RFC, and according to #8212 , the following code is deprecated in PHP 8.2, and is scheduled for removal in PHP 9.0:

<?php
var_dump("${foo}");
// Deprecated: Using ${} in strings is deprecated
 
var_dump("${(foo)}");
// Deprecated: Using ${} (variable variables) in strings is deprecated

However, this RFC does not document what the behaviour will be in PHP 9.0. Will it raise a syntax error? Or will treat it like a plain string literal?

<?php
var_dump("${foo}");
// In PHP 9.0, will this raise a syntax error? Or will it be equal to '${foo}' ?
 
var_dump("${(foo)}");
// In PHP 9.0, will this raise a syntax error? Or will it be equal to '${(foo)}' ?

I think this is important to consider this question to help those migrating their codebase to PHP 9.0 in the future, and to alert those who may not realise this code may need modification. I personally propose that this raise a syntax error in PHP 9.0. Later versions can allow this syntax again, if needed, with the new behaviour of acting like a normal string literal with no special behaviour. This means that the migration path would be:

1. Old syntax is supported
2. Old syntax raises a deprecation warning, but works as before (PHP 8.2)
3. Old syntax raises a syntax error (PHP 9.0)
4. In the future, the old syntax may be allowed again, with the new behaviour

PHP Version

PHP 8.2

Operating System

No response

[iluuu1994]

Hm, you're right, this should've been stated more clearly. The plan is to raise an error, in case we ever want to re-use the syntax without another BC break. And as you stated, that will make it more obvious that the syntax is no longer supported if somebody where to migrate directly from PHP 8.1 to 9.0. I'll send an e-mail to the mailing list to clarify.

[alecpl]

I'm not sure that it should be an error. For example echo "$."; will print $., so expectation on "${" might be the same. In the upgrading context an error would probably be better, but I see some inconsistency here.

The fact that the RFC did not say that it will be an error is also a reason to not make it an error.

[iluuu1994]

I don't see how ${ and $. are connected but being pedantic, other cases like "{$}" produce errors too. Erroring for ${} will improve the migration path and keep the syntax a possibility for later. Those are two very strong arguments, I don't see any good arguments for the contrary.

[alecpl]

My point was that $ without the following variable name is just a dollar sign (does not throw an error). ${ "combination" is the same as $. or $# or $!, etc. and should/could be treated the same. I'm pretty sure it would be treated this way if there was no history of ${} being used for something. Of course, you can say that because {$var} exists we should treat ${var} as a typo, but I'm not sure this is good enough reason to throw an error. Imo, the case is not obvious.

[Flimm]

When PHP 9.0 comes out, there will be three groups of PHP users:

1. Those who expect that "${var}" will behave just like "$var" as it always has done in PHP.
2. Those who know the behaviour of "${var}" is changing, and have read the documentation about it and have adjusted their code to match new documented behaviour
3. Those who realise that "${var}" is changing, but haven't read the documentation completely and make an assumption about the way that "${var}" is changing, possibly a false assumption

I think that group 1 and group 2 have many more users that group 3.

To cater to group 1, we should make "${var}" a syntax error at parse time in PHP 9.0, so that these developers experience loud failures of their code, rather than silent failures, which are much harder to catch and troubleshoot. To cater to group 2, we should document the new behaviour clearly. Syntax errors are good even for group 3, because those users will realise quicker that their assumptions about the new behaviour are false (even if group 3 would have preferred the new behaviour to match their assumptions). But in any case, group 1 and group 2 are more important because they are much larger than group 3, in my opinion.

It's also worth remembering that a future version of PHP could give new behaviour to "${var}" if the developers wanted to do that (even if it's just making it the same as '${var}'). Making "${var}" a syntax error in PHP 9.0 opens the door for new behaviour in later versions of PHP.