utf8ストリングのcharを一つ一つ読んでいきたい場合、charsをarrayにsplitするほうが、
mb_substrでストリングの最初から探すほうが効率がいい。

しかし、splitより効率がいい方法は、現在のpointerからnext charを見つける
functionを使うことである。

https://stackoverflow.com/questions/3666306/how-to-iterate-utf-8-string-in-php

=========================================================================================
Preg split will fail over very large strings with a memory exception and mb_substr is slow indeed, so here is a simple, and effective code, which I'm sure, that you could use:

function nextchar($string, &$pointer){
    if(!isset($string[$pointer])) return false;
    $char = ord($string[$pointer]);
    if($char < 128){
        return $string[$pointer++];
    }else{
        if($char < 224){
            $bytes = 2;
        }elseif($char < 240){
            $bytes = 3;
        }else{
            $bytes = 4;
        }
        $str =  substr($string, $pointer, $bytes);
        $pointer += $bytes;
        return $str;
    }
}
This I used for looping through a multibyte string char by char and if I change it to the code below, the performance difference is huge:

function nextchar($string, &$pointer){
    if(!isset($string[$pointer])) return false;
    return mb_substr($string, $pointer++, 1, 'UTF-8');
}
Using it to loop a string for 10000 times with the code below produced a 3 second runtime for the first code and 13 seconds for the second code:

function microtime_float(){
    list($usec, $sec) = explode(' ', microtime());
    return ((float)$usec + (float)$sec);
}

$source = 'árvíztűrő tükörfúrógépárvíztűrő tükörfúrógépárvíztűrő tükörfúrógépárvíztűrő tükörfúrógépárvíztűrő tükörfúrógép';

$t = Array(
    0 => microtime_float()
);

for($i = 0; $i < 10000; $i++){
    $pointer = 0;
    while(($chr = nextchar($source, $pointer)) !== false){
        //echo $chr;
    }
}

$t[] = microtime_float();

echo $t[1] - $t[0].PHP_EOL.PHP_EOL;
=========================================================================================
このコードからわかるように、utf8のキャラのByteサイズを判定する方法である。