declare(encoding=...) ignored because Zend multibyte feature is turned off by settings for ASCII or UTF-8

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:

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:

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=1 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 for 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

No source file or test is named. Reproduce the warning with zend.multibyte set to 0 and 1 using the listed declare(encoding=...) values, then determine the intended warning behavior; done means the agreed behavior is implemented and covered by regression checks.