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

Description

According to this accepted RFC, and according to https://github.com/php/php-src/pull/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