Modx Revo plugin, strip content and set it to introtext

modx-plugin-content-introtext.php

<?php
//OnBeforeDocFormSave
$introlength = $modx->getOption('IntrotextOptions',$scriptProperties,'400');
$introtext= $resource ->introtext; 
$text =$modx->stripTags($resource->content);

if(empty($introtext)){
  if (strlen($text) > $introlength) {
    $cuttext = mb_substr($text, 0, $introlength,"UTF-8");
    $pos = mb_strrpos($cuttext, ' ',"UTF-8");
    $output = mb_substr($cuttext, 0, $pos,"UTF-8");
  }
  else{
    $output = $text;
  }
  $resource->set('introtext',$output);
}

$resource->save();

//$modx->event->output('внимание: '.$pos);