使用php从数据库中选择两个值，然后传递给android studio

Question

这是我的android studio的代码

 Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                String result = null;
                InputStream is = null;

                List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
                nameValuePairs.add(new BasicNameValuePair("username", username));
                nameValuePairs.add(new BasicNameValuePair("friend", friend));
                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost("http://192.168.1.101/webservice/gethismessage.php");
                httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
                HttpResponse response = httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();
                is = entity.getContent();
                BufferedReader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"), 8);
                StringBuilder sb = new StringBuilder();
                String line = null;
                while ((line = reader.readLine()) != null) {

                    sb.append(line + "\n");
                }
                is.close();
                result = sb.toString();


                JSONArray jArray = new JSONArray(result);
                String[] usernameq = new String[jArray.length()];
                String[] message = new String[jArray.length()];
                for (int i = 0; i < jArray.length(); i++) {
                    usernameq[i] = jArray.getJSONObject(i).getString("sentuser");
                    message[i] = jArray.getJSONObject(i).getString("message");
                    if (usernameq[i] == username) {
                        ChatMessage chatMessage = new ChatMessage().setLocal(true).setsentuser(true).setContent(message[i]);
                        addMessage(chatMessage);
                    } else {
                        ChatMessage chatMessage = new ChatMessage().setLocal(true).setsentuser(false).setContent(message[i]);
                        addMessage(chatMessage);
                    }

                }

这是我的PHP代码

<?php
$username ='root';

$password ='lean96022198';

$hostname ='localhost';

$database ='test_xmpp';


$localhost = mysql_connect($hostname,$username,$password) or 
trigger_error(mysql_error(),E_USER_ERROR);

mysql_select_db($database,$localhost);
$username = $_POST["username"]
$friend = $_POST["friend"]
$i=mysql_query("select sentuser,message from messagehistory where (sentuser= $username and        receiveuser= $friend) or (sentuser= $friend and receiveuser= $username)");
$num_rows = mysql_num_rows($i);
while($row = mysql_fetch_array($i))
$r[]=$row;
print(json_encode($r));
echo"hello"
?>

我在这两个脚本中出错

usernameq[i] = jArray.getJSONObject(i).getString("sentuser");
                    message[i] = jArray.getJSONObject(i).getString("message");

请帮我修复我的php或android代码的脚本，以便我可以获得＆＃34; sentuser＆＃34;的值。和＆＃34;消息＆＃34;成功地。我只知道如何从db中选择一个值来获取值。