Substr_replace и utf-8

if (function_exists('mb_substr_replace') === false)
{
    function mb_substr_replace ($string, $replacement, $start, $length = 0)
    {
        if (is_array($string))
        {
            foreach ($string as $i => $val)
            {
                $repl = is_array ($replacement) ? $replacement[$i] : $replacement;
                $st   = is_array ($start) ? $start[$i] : $start;
                $len  = is_array ($length) ? $length[$i] : $length;
 
                $string[$i] = mb_substr_replace ($val, $repl, $st, $len);
            }
 
            return $string;
        }
 
        $result  = mb_substr ($string, 0, $start, 'UTF-8');
        $result .= $replacement;
 
        if ($length > 0) {
            $result .= mb_substr ($string, ($start+$length+1), mb_strlen($string, 'UTF-8'), 'UTF-8');
        }
 
        return $result;
    }
}

<?php
 
function replace($string, $pos, $char, $encoding = "UTF-8")
{
    return mb_substr($string, 0, $pos, $encoding)
        .$char
        .mb_substr($string, $pos+1, NULL, $encoding);
}
 
$string = "Привет";
$encoding = "UTF-8";
$length = mb_strlen($string, $encoding);
for ($i = 0; $i < $length; $i++)
    echo ($i+1)." - ".replace($string, $i, "*")."<br>\n";