$words = array('поставя','темата', 'продавал'); // Allowed words
$content = file_get_contents('https://www.wallflux.com/feed/101392669928073'); // Feed url
$xml = new SimpleXmlElement($content);
foreach($xml->channel->item as $row) {
    foreach($words as $word) {
        if(strpos($row->description, $word) !== false) {
            /*
            Check in DB from $row->pubDate
            False = send email (url to post - $row->link), write on DB
            True =  continue
            */
        }
    }
}