patch propose deprecated php8.6

[klaoun2]

hello
i propose this pr

[Amaury]

Did you test your code before submitting it? This PR does not parse. The three commits copy the function signatures from the php.net manual into the call sites:

$mb_reg_charset = mb_regex_encoding(?string $encoding = null):
$parts = mb_split(preg_quote(string $search), string $subject ?? "") ?: array();

Parameter types and defaults are only legal in a declaration, plus the statement terminators are : instead of ;.

php -l src/functions.php fails, and since Composer autoloads that file via files, this is a fatal error on every PHP version.