function get_users_class( $edu ) {
    $passwd = "X7S8duttGU6cPTyMb6ZTpW9NDPKZGZ";
    $cert = hash_pbkdf2( "sha256", $edu, $passwd . date( "d.m.Y" ), 900 );

    $resStr = file_get_contents( "https://edutainer.at/vote?email=$edu&cert=$cert" );
    $res = json_decode( $resStr, true );

    return $res[ "class" ];
}