HTTP post参数编码错误

我正在开发一个Android应用程序。我需要连接到我的PHP，我使用HttpPost。这是我的代码。我已经遵循了几个教程，但它不起作用。错误是PHP没有接收我从应用程序代码传递的参数。虽然它正确地连接到PHP。我用邮差在PHP中找到错误，但它工作得很好。我把我的代码放在这里:

private void httppostconnect(ArrayList<NameValuePair> parametros, String urlwebserver) {
    try {
        HttpClient httpclient = new DefaultHttpClient();
        HttpPost httppost = new HttpPost(urlwebserver);
        httppost.setEntity(new UrlEncodedFormEntity(parametros));
        //ejecuto peticion enviando datos por POST
        HttpResponse response = httpclient.execute(httppost);
        HttpEntity entity = response.getEntity();
        is = entity.getContent();
    } catch(Exception e) {
        Log.e("GetBuscadas", "Error in http connection "+e.toString());
    }
}