I agree that some people might want a mb_explode('', $string);
this is my solution for it:
<?php
$string = 'Hallöle';
$array = array_map(function ($i) use ($string) {
return mb_substr($string, $i, 1);
}, range(0, mb_strlen($string) -1));
expect($array)->toEqual(['H', 'a', 'l', 'l', 'ö', 'l', 'e']);
?>