PHP:未定义的索引错误，但变量在打印时具有正确的值

我正在将一些值从android应用程序传递到PHP脚本。我在PHP脚本中得到了一个未定义的索引错误，但当我从脚本中打印变量时，它们的值是正确的。我希望这些错误消失，但我一开始就不明白为什么会出现。以下是如何将它们传递到PHP脚本。

Java代码

//build url data to be sent to server
List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
nameValuePairs.add(new BasicNameValuePair("username",username));
nameValuePairs.add(new BasicNameValuePair("password",password));
String result = "";
InputStream is = null;
//http post
try{
  HttpClient httpclient = new DefaultHttpClient();
  HttpPost httppost = new HttpPost("http://10.0.2.2/PasswordCheck.php");
  httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs,"utf-8"));
  HttpResponse response = httpclient.execute(httppost);
  HttpEntity entity = response.getEntity();
  is = entity.getContent();
}catch(Exception e){
  Log.e("Connection", "Error in http connection "+e.toString());
}

PHP代码：

<?php
mysql_connect("localhost", "root", "") or die("could not connect to mysql");
mysql_select_db("drop-in") or die("database not found");
if(isset($_POST["username"])){
    $username = $_POST["username"];
}
if(isset($_POST["password"])){
    $suppliedPassword = $_POST["password"];
}
$databasePassword = "";
$output = "false";
$query = mysql_query("SELECT Password FROM users WHERE Username = '$username'") or die("query failed");
if(mysql_num_rows($query) > 0){
    $row = mysql_fetch_assoc($query); 
    $databasePassword = $row['password'];
    if($databasePassword == $suppliedPassword)
    {
        $output = "true";
    }
}
    print($output);
    mysql_close();
?>

EDIT：添加了PHP脚本（它们不在同一个文件中，代码标记行为不端）