echo truncate('jonas', 3, '...'); //jo<...
echo truncate('jonas', 3, '...', true); //jon...
echo truncate('jonas', 3, '...', true, false); //jon...

Segue la funzione:

"]function truncate($s, $l, $e = '...', $isHTML = false){
  $i = 0;
  $tags = array();
  if($isHTML){
    preg_match_all('/<[^>]+>([^<]*)/', $s, $m, PREG_OFFSET_CAPTURE | PREG_SET_ORDER);
    foreach($m as $o){
      if($o[0][1] - $i >= $l)
        break;
      $t = substr(strtok($o[0][0], " \t\n\r\0\x0B>"), 1);
      if($t[0] != '/')
        $tags[] = $t;
      elseif(end($tags) == substr($t, 1))
        array_pop($tags);
      $i += $o[1][1] - $o[0][1];
    } 
  } 
  return substr($s, 0, $l = min(strlen($s),  $l + $i)) .   (count($tags = array_reverse($tags)) ? '' : '') .   (strlen($s) > $l ? $e : '');
}