fixed title.

As these functions deal with multibyte strings, they naturally belong into the mbstring extension, which, as you mention, already has analogous functions. Why would these need to be mirrored into ext/standard?

@nikic

The one of target is Symfony's polyfill-mbstring and Drupal adopt them.

https://github.com/symfony/polyfill
https://api.drupal.org/api/drupal/vendor!symfony!polyfill-mbstring!Mbstring.php/class/Mbstring/8.2.x

Algthouh mb_strlen and mb_substr can properly handle ill-formed byte sequences, PCRE's functions iconv_strlen/iconv_substr can't handle them. These functions are used for emulating mbstring's functions. grapheme_strlen and grapheme_substr also cant't handle them.

$str = "ab\x80cde";

var_dump(
    6 === mb_strlen($str, 'UTF-8'),
    false === preg_match_all('/./su', $str),
    false === iconv_strlen($str, 'UTF-8'),
    NULL === grapheme_strlen($str)
);

iconv cant't handle ill-formed byte properly even in 2016.

https://sourceware.org/bugzilla/show_bug.cgi?id=2373

I will open other requests for adding encoding option to substr (chr and ord's pull requests exist) and new functions (str_at and str_codepoint_at and str_each_char and str_each_codepoint) and rethink the value of this feature request.

See github thread: https://github.com/php/php-src/pull/2129#issuecomment-270061348