Funzione di troncamento compatibile con UTF-8

Potresti usare mb_strimwidth (Documentazione PHP):

echo mb_strimwidth("Hello World", 0, 10, "...");

O una funzione personalizzata come Multibyte Tronca stringa per Smarty :

mb_truncate($string, $length = 80, $etc = '...', $charset='UTF-8',
                                  $break_words = false, $middle = false)
{
    if ($length == 0)
        return '';

    if (strlen($string) > $length) {
        $length -= min($length, strlen($etc));
        if (!$break_words && !$middle) {
            $string = preg_replace('/\s+?(\S+)?$/', '', mb_substr($string, 0, $length+1, $charset));
        }
        if(!$middle) {
            return mb_substr($string, 0, $length, $charset) . $etc;
        } else {
            return mb_substr($string, 0, $length/2, $charset) . $etc . mb_substr($string, -$length/2, $charset);
        }
    } else {
        return $string;
    }
}