I am thinking if the mb_strpos() only when needle is exact string and has multibyte, the original str_contains() to strpos() already converted on DowngradeStrContainsRector, so should be a logic to verify that the needle has multibye, eg:

strlen($needle) !== mb_strlen($needle)

this, however, I prefer to not be part of downgrade php 8.0 set, as dealing with multibyte can be "there on purpose", user that needs exact change str_contains -> mb_strpos needs to enable manually as use and know the risk.

Only when needle has multibyte value as well:

Only when needle has multibyte value as well:

Ok

these fixtures are invalid, as no multibyte in needle, only downgrade to mb_strpos when needle has multibyte char

$isMatch = !str_contains('😊abc', 😊'a');

unregister this, there is no "mb_str_contains", only str_contains, that means that utilize strpos can be on purpose.

let user manually use if needed, dealing with user that may don't have mbstring extension or use strpos on purpose :)

user that use mb_ functions know why they are using the mb_ functions :)

early skip first class callable here:

if ($funcCall->isFirstClassCallable()) {
    return null;
}

if (!$funcCall->isFirstClassCallable()) {
  return null;
}

Correct ?

no, should be below if (! $funcCall instanceof FuncCall) {:

        if (! $funcCall instanceof FuncCall) {
            return null;
        }

        if ($funcCall->isFirstClassCallable()) {
            return null;
        }

Add test fixture to skip no multibyte needle:

$isMatch = str_contains('abc', 'a');

Added the string without multibyte needle.

skip first class callable haystack as well.

I prefer to rename to something like: DowngradeStrContainsWithMultibyteNeedleRector since there is no mb_str_contains, make clearer it actually use when needle has multibyte.

Ok