mb_substr_replace для работы с utf-8
<?
function mb_substr_replace($original, $replacement, $position, $length)
{
$startString = mb_substr($original, 0, $position, "UTF-8");
$endString = mb_substr($original, $position + $length, mb_strlen($original), "UTF-8");
$out = $startString . $replacement . $endString;
return $out;
}