Issue: https://github.com/php/php-src/issues/8501
Author: Flimm
### Description
According to[ this accepted
RFC](https://wiki.php.net/rfc/deprecate_dollar_brace_string_interpolation), 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
<?php
var_dump("${foo}");
// Deprecated: Using ${} in strings is deprecated
var_dump("${(foo)}");
// Deprecated: Using ${} (variable variables) in strings is deprecated
```
However, this RFC does not document what the behaviour will be in PHP 9.0. Will it raise a syntax
error? Or will treat it like a plain string literal?
```php
<?php
var_dump("${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_