I needed to cut a string after x chars at a  html converted utf-8 text (for example Japanese text like 嬰謰弰脰欰罏). 
The problem was, the different length of the signs, so I wrote the following function to handle that.
Perhaps it helps.

<?php

function html_cutstr ($str, $len)
{
    if (!preg_match('/\&#[0-9]*;.*/i', $str))
    {
        $rVal = strlen($str, $len);
        break;
    }

    $chars = 0;
    $start = 0;
    for($i=0; $i < strlen($str); $i++)
    {
        if ($chars >= $len)
        break;

        $str_tmp = substr($str, $start, $i-$start);
        if (preg_match('/\&#[0-9]*;.*/i', $str_tmp))
        {
            $chars++;
            $start = $i;
        }
    }
    $rVal = substr($str, 0, $start);
    if (strlen($str) > $start)
    $rVal .= " ...";
    return $rVal;
}
?>