<?php

$text = "äöü &auml; &ouml; &uuml; &#223;";
var_dump(mb_convert_encoding($text, 'UTF-8', 'HTML-ENTITIES'));

$text = "äöü &auml; &ouml; &uuml; &#223;";
var_dump(html_entity_decode($text, ENT_COMPAT | ENT_HTML401, 'UTF-8'));