如何在PHP中解码UTF-8中的特殊字符

I am having a html with special HTML attributes which are not supporting in html5

currently, I am using

html_entity_decode(htmlspecialchars($html, ENT_QUOTES | ENT_HTML5 | ENT_DISALLOWED | ENT_SUBSTITUTE, 'UTF-8'));

for decoding special attributes. But all the special attributes are converted like this-

Just reply to this email and let us know the reason � We would really appreciate it! Thank you.

I have tried this one also link

I have already wasted one day. Any help would be greatly appreciated. Thanks in advance.