[php-src] Issue #21538: declare(encoding=...) ignored because Zend multibyte feature is turned off by settings for ASCII or UTF-8

Issue: https://github.com/php/php-src/issues/21538
Author: LLyaudet

### Description

Hello :),

Assume you mix files in distinct encodings in various projects.
If some are in ISO-8859-1 or ISO-8859-15 for example,
and others are in UTF-8.
You can have the habit to always use:
```php
declare(encoding='ISO-8859-1')
// or
declare(encoding='ISO-8859-15')
// or
declare(encoding='UTF-8')
// or
declare(encoding='ASCII')
```
However, if `zend.multibyte=0` and you use files with:
```php
declare(encoding='UTF-8')
// or
declare(encoding='ASCII')
```
you will get
```
PHP Warning:  declare(encoding=...) ignored because Zend multibyte feature is turned off by settings in
```
which makes you want to remove them.

And if `zend.multibyte=0` and you use files with all declare(encoding=...)
you will get no warning.

I would prefer that PHP states clearly what is preferred when the files can be used separately,
and hence some of these files without Zend multibyte:
- If the prefered solution is to always have the declare(encoding=...) directive,
  then the warning
  ```
  PHP Warning:  declare(encoding=...) ignored because Zend multibyte feature is turned off by settings in
  ```
  should be suppressed with encodings that do not require multibyte.
- If the prefered solution is to never have the declare(encoding=...) directive for encodings that don't need it,
  then the warning should be distinct:
  ```
  PHP Warning:  declare(encoding='UTF-8') is useless, remove it.
  ```
  and appear whatever the value of Zend multibyte configuration.

Thanks, best regards,
    Laurent Lyaudet