rss-bridge/formats/PlaintextFormat.php

<?php

class PlaintextFormat extends FormatAbstract
{
    const MIME_TYPE = 'text/plain';

    public function stringify()
    {
        $data = [];
        foreach ($this->getItems() as $item) {
            $data[] = $item->toArray();
        }
        $text = print_r($data, true);
        // Remove invalid non-UTF8 characters
        ini_set('mbstring.substitute_character', 'none');
        $text = mb_convert_encoding($text, $this->getCharset(), 'UTF-8');
        return $text;
    }
}