Description:
------------
I propose mb_at and mb_codepoint_at.

string mb_at ( string $string , int $offset [, string $encoding = mb_internal_encoding() ] )
int mb_codepoint_at ( string $string , int $offset [, string $encoding = mb_internal_encoding() ] )

The one of usage of these functions is to iterate over all the chars in string.

$str = "Cat!\u{1f431}";

$gen = (function($str) {
    for ($i = 0, $len = mb_strlen($str); $i < $len; ++$i) {
        yield mb_at($str, $i);
    }
})($str);

foreach ($gen as $char) {
    echo $char, PHP_EOL;
}

mb_codepoint_at is equivalent to the following code:

mb_ord(mb_substr($str, $i, 1));

str_at and str_codepoint_at are worthy of consideration.