How to iterate a UTF-8 string character by character using indexing?

When you access a UTF-8 string with the bracket operator $str[0] the utf-encoded character consists of 2 or more elements.

For example:

$str = "Kąt"; $str[0] = "K"; $str[1] = "�"; $str[2] = "�"; $str[3] = "t";

but I would like to have:

$str[0] = "K"; $str[1] = "ą"; $str[2] = "t";

It is possible with mb_substr but this is extremely slow, ie.

mb_substr($str, 0, 1) = "K" mb_substr($str, 1, 1) = "ą" mb_substr($str, 2, 1) = "t"