Multibyte substr_replace(). The mbstring library doesn’t come with a multibyte equivalent of substr_replace(). This function behaves exactly like substr_replace() even when the arguments are arrays.

mb_substr_replace.php

<?php
function mb_substr_replace($string, $replacement, $start, $length=NULL) {
    if (is_array($string)) {
        $num = count($string);
        // $replacement
        $replacement = is_array($replacement) ? array_slice($replacement, 0, $num) : array_pad(array($replacement), $num, $replacement);
        // $start
        if (is_array($start)) {
            $start = array_slice($start, 0, $num);
            foreach ($start as $key => $value)
                $start[$key] = is_int($value) ? $value : 0;
        }
        else {
            $start = array_pad(array($start), $num, $start);
        }
        // $length
        if (!isset($length)) {
            $length = array_fill(0, $num, 0);
        }
        elseif (is_array($length)) {
            $length = array_slice($length, 0, $num);
            foreach ($length as $key => $value)
                $length[$key] = isset($value) ? (is_int($value) ? $value : $num) : 0;
        }
        else {
            $length = array_pad(array($length), $num, $length);
        }
        // Recursive call
        return array_map(__FUNCTION__, $string, $replacement, $start, $length);
    }
    preg_match_all('/./us', (string)$string, $smatches);
    preg_match_all('/./us', (string)$replacement, $rmatches);
    if ($length === NULL) $length = mb_strlen($string);
    array_splice($smatches[0], $start, $length, $rmatches[0]);
    return join($smatches[0]);
}