Alternative to mb_convert_encoding with HTML-ENTITIES charset

I have the following code:

mb_convert_encoding($string, 'HTML-ENTITIES', 'utf-8');

I need to have an alternative code which does exactly the same but does not use any mb_* functions (the mb extension is not available on some environments).

I thought that

utf8_decode(htmlentities($string, ENT_COMPAT, 'utf-8'));

should do exactly the same, but unfortunately it does not.