I do this all the time: Call a PHP page with WWW to get some data from a Database. Now whenever I tried to return characters that were not a part of the ASCII printable characters, I’d get ?? questionmarks in Unity.
The trick is to make sure the text is properly encoded before you echo it!
echo “ÄäÜüÖö”; //does not work :(
echo mb_convert_encoding(“ÄäÜüÖö”, ‘UTF-8′); //works!
More info on the PHP help pages