<?php
function addContent($addTo, $add, $cntRep, $after = '</p>', $openTag = '<div align="center">', $closeTag = '</div>'){
$posAfter = 0;
for ($i=0; $i < $cntRep; $i++) {
$pos = stripos($addTo, $after, $posAfter);
$posAfter = $pos+strlen($after);
if($pos===FALSE){
return $addTo;
}
}
$beforeAdding = substr($addTo, 0, $posAfter);
$afterAdding = substr($addTo, $posAfter);
$result = $beforeAdding . $openTag . $add . $closeTag . $afterAdding;
return $result;
}

$this->item->fulltext = addContent($this->item->fulltext,'<div> ВАШ КОД </div>',5);

echo $this->item->fulltext;
?>