I don't know if this function is multibyte safe but I've written a function that will do the same in multibyte mode.

<?php
//Check to see if it exists in case PHP has this function later
if (!function_exists("mb_substr_replace")){
   //Same parameters as substr_replace with the extra encoding parameter.
    function mb_substr_replace($string,$replacement,$start,$length=null,$encoding = null){
        if ($encoding == null){
            if ($length == null){
                return mb_substr($string,0,$start).$replacement;
            }
            else{
                return mb_substr($string,0,$start).$replacement.mb_substr($string,$start + $length);
            }
        }
        else{
            if ($length == null){
                return mb_substr($string,0,$start,$encoding).$replacement;
            }
            else{
                return mb_substr($string,0,$start,$encoding). $replacement. mb_substr($string,$start + $length,mb_strlen($string,$encoding),$encoding);
            }
        }
    }
}
?>