How can I know whether the value posted in c# reached PHP

Question

I am a beginner in C#, I am creating a windows phone app in which login page requests validation from remote url's PHP file ,I can get the response from the PHP file but I dont know how to check ,Whether the value(Username,Password) is Posted to PHP file

Program

private async void Button_Click(object sender, RoutedEventArgs e)
{
    var username = usernames .Text;
    var password = passwords .Password;

    var postMessage = new StringContent(string.Format("username={0}&password={1}", username, password), Encoding.UTF8 , "application/x-www-form-urlencoded");

    var response = await (new HttpClient()).PostAsync("http://xxx.xxxx.xxx/xxx.php", postMessage);
} 

I want to Know how to post and receive value from c# to php