<?php
$string = '<div>ěščřžýáíé</div><p>testik</p> <p>testik2</p>';
$dom = new \DOMDocument('1.0', 'UTF-8');
$dom->loadHTML('<body>'.mb_convert_encoding($string, 'HTML-ENTITIES', 'UTF-8').'</body>');
// změny na $dom
echo substr($dom->saveHTML($dom->documentElement), 12, -14);