LastFM track.updateNowPlaying in PHP

Question

To be able to change the now playing track you have to send "api_sig" as a post parameter in your request (see in the last.fm documentation). I am building this string as described in the documentation, but still, I always get an error: "Invalid Method - No method with that name in this package". However, I don't think, that the method doesn't exist, I think, that my api_sig is not correct as there's no error code for a wrong api_sig and I can't update other methods (f.e. track.scrobble).

This is my code so far (I know it's not very clean.. will get changed as soon as it works :D):

 $url = 'http://ws.audioscrobbler.com/2.0/?api_key='.$apikey.'&format=json';
    $params = array(
        "api_key" => $apikey,
        "method" => "track.updateNowPlaying",
        "artist" => "eiffel65",
        "track" => "Blue",
        "sk" => $sessionkey
    );
    ksort($params);
    $sigstr = "";
    foreach ($params as $x => $x_value) {
        $sigstr .= $x.$x_value;
    }
    $params = array(
        "api_key" => urlencode($apikey),
        "method" => urlencode("track.updateNowPlaying"),
        "artist" => urlencode("eiffel65"),
        "track" => urlencode("Blue"),
        "sk" => urlencode($sessionkey)
    );
    $sigstr .= $apisecret;
    $sigstr = md5(utf8_encode($sigstr));

    $curl = curl_init();
    $headers = array();
    $params["api_sig"] = $sigstr;

    curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($curl, CURLOPT_POSTFIELDS, $params);
    curl_setopt($curl, CURLOPT_HEADER, 0);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_TIMEOUT, 30);
    $json = curl_exec($curl);
    curl_close($curl);

I'd appreciate any tips and/or code snippets to help me out with my issue!