$html = 'hello!';
$document = new DOMDocument();
$document->loadHTML(mb_convert_encoding($html, 'HTML-ENTITIES', 'UTF-8'));
echo $document->saveHTML();<html><body><p>Добрый день!</p></body></html>
<?php
$html = 'Добрый день!';
$document = new DOMDocument();
$document->loadHTML(mb_convert_encoding($html, 'HTML-ENTITIES', 'UTF-8'));
echo $document->saveHTML((new \DOMXPath($document))->query('/')->item(0));