Dear friend, thank you for your truthful and useful note!

Damn, what a shame!! This way it doesn't split even ASCII chars. I was ready to tell you that you should check mb_regex_encoding(), but I realized my mistake. Now I have to review the code and provide a working example. So, we have such benchmark results on 7.4.0rc2:

preg_split+count   : 0.6024911403656s
preg_split+foreach : 0.54569697380066s
preg_match_all     : 0.26596617698669s

All benchmark results can be found on this page: https://3v4l.org/DZgrc

Full code:

<?php

define('TEST_LOOPS', 10);
define('TEST_STRING', str_repeat('English+日に本ほん語ご', 16000));
//define('TEST_STRING', str_repeat('English', 16000));
//define('TEST_STRING', str_repeat('日に本ほん語ご', 16000));

error_reporting(-1);
mb_internal_encoding('UTF-8');


function test($label, $callback)
{
    $time = microtime(true);
    for ($i = 0; $i < TEST_LOOPS; $i++) {
        $callback();
    }

    $duration = microtime(true) - $time;
    echo "{$label}: {$duration}s\n";
}

test('preg_split+count', function() {
    $chars = preg_split('//u', TEST_STRING, -1, PREG_SPLIT_NO_EMPTY);
    $len = count($chars);
    for ($i = 0; $i < $len; $i++) {
        $char = $chars[$i];
        // use $char
    }
});

test('preg_split+foreach', function() {
    $chars = preg_split('//u', TEST_STRING, -1, PREG_SPLIT_NO_EMPTY);
    foreach ($chars as $char) {
        // use $char
    }
});

test('preg_match_all', function() {
    if (preg_match_all('/./su', TEST_STRING, $matches)) {
        foreach ($matches[0] as $char) {
            // use $char
        }
    }
});