I created some functions for entity-safe splitting+lengthcounting:

<?php
function strlen_entities($text)
{
    preg_match_all(
        '/((?:&(?:#[0-9]{2,}|[a-z]{2,});)|(?:[^&])|'.         
        '(?:&(?!\w;)))s',$text,$textarray);
    return count($textarray[0]);
}  
function substr_entities($text,$start,$limit=0)
{
    $return = '';
    preg_match_all(
        '/((?:&(?:#[0-9]{2,}|[a-z]{2,});)|(?:[^&])|'.         
        '(?:&(?!\w;)))s',$text,$textarray);
    $textarray = $textarray[0];
    $numchars = count($textarray)-1;
    if ($start>=$numchars)
        return false;
    if ($start<0)
    {
        $start = ($numchars)+$start+1;
    }
    if ($start>=0)
    {
        if ($limit==0)
        {
            $end=$numchars;
        }
        elseif ($limit>0)
        {
            $end = $start+($limit-1);
        }
        else
        {
            $end = ($numchars)+$limit;
        }

        for ($i=$start;$i<=$end;$i++)
        {
            $return .= $textarray[$i];
        }
        return $return;
    }
}
?>