Hmm ... this is a script I wrote, whitch is very similar to substr, but it isn't takes html and bbcode for counting and it takes portion of string and show avoided (html & bbcode) tags too ;] 
Specially usefull for show part of serach result included html and bbcode tags

<?php

/**
 * string csubstr ( string string, int start [, int length] )
 *
 * @author FanFataL
 * @param string string
 * @param int start
 * @param [int length]
 * @return string
 */
function csubstr($string, $start, $length=false) {
    $pattern = '/(\[\w+[^\]]*?\]|\[\/\w+\]|<\w+[^>]*?>|<\/\w+>)/i';
    $clean = preg_replace($pattern, chr(1), $string);
    if(!$length)
        $str = substr($clean, $start);
    else {
        $str = substr($clean, $start, $length);
        $str = substr($clean, $start, $length + substr_count($str, chr(1)));
    }
    $pattern = str_replace(chr(1),'(.*?)',preg_quote($str));
    if(preg_match('/'.$pattern.'/is', $string, $matched))
        return $matched[0];
    return $string;
}

?>

Using this is similar to simple substr.

Greatings ;]
...