ho provato

mb_convert_encoding(urlencode($stringa), "UTF-8")
mb_convert_encoding(urldecode($stringa), "UTF-8")
mb_convert_encoding(urlencode($stringa), "ISO-8859-1")
mb_convert_encoding(urldecode($stringa), "ISO-8859-1")

ma niente !

ritorna

%3F
?
%3F
?