Use the following to decode all entities:
<?php html_entity_decode($string, ENT_QUOTES | ENT_XML1, 'UTF-8') ?>

I've checked these special entities: 
- double quotes (&#34;)
- single quotes (&#39; and &apos;) 
- non printable chars (e.g. &#13;)
With other $flags some or all won't be decoded.

It seems that ENT_XML1 and ENT_XHTML are identical when decoding.