ต้องการส่งค่าตัวแปร จาก Android ไป php เพื่อนำไป WHERE ฐานข้อมูล

public class SelectsubjActivity extends ListActivity {
	
	LinearLayout layout;

	public static final String url = "http://checkstd.w.pw/userlist.php";
	public static String result = "";
	List<User> subj = new ArrayList<User>();
	
	private String text;
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_selectsubj);
		
		Bundle bundle = getIntent().getExtras();
		text = bundle.getString("mmuser");
		
		TextView tuser = (TextView) findViewById(R.id.user);
		tuser.setText(text);		
		
		WebPageTask task = new WebPageTask();
		task.execute();
	}

	@Override
	protected void onListItemClick(ListView l, View v, int position, long id) {
	/*	User u = subj.get(position);
		final Dialog dialog = new Dialog(SelectsubjActivity.this);
		LinearLayout linearLayout = new LinearLayout(this);
		linearLayout.setPadding(10, 10, 10, 10);
		TextView txt = new TextView(this);
		txt.setText(u.subj_id);
		linearLayout.addView(txt);
		dialog.setContentView(linearLayout, new LayoutParams(400,
				LinearLayout.LayoutParams.WRAP_CONTENT));
		dialog.setTitle("User Detail");
		dialog.setCancelable(true);
		//dialog.show();
		
		Intent i = new Intent(SelectsubjActivity.this, SelectsecActivity.class);
		i.putExtra("d_subj", "txt");
		startActivityForResult(i, 1);*/
		
		startActivity(new Intent(SelectsubjActivity.this, SelectsecActivity.class));
	}
	
	public void showAllUser() {

		ArrayAdapter<User> adapter = new ArrayAdapter<User>(this,
				android.R.layout.simple_list_item_1, subj);
		setListAdapter(adapter);
	}

	public String getData(String url, List<NameValuePair> nameValuePairs) {
		HttpClient httpclient = new DefaultHttpClient();
		HttpPost httppost = new HttpPost(url);
		BufferedReader in = null;
		try {    
			HttpResponse response = httpclient.execute(httppost);
			in = new BufferedReader(new InputStreamReader(response.getEntity()
					.getContent()));
			StringBuffer sb = new StringBuffer("");
			String line = "";
			String NL = System.getProperty("line.separator");
			while ((line = in.readLine()) != null) {
				sb.append(line + NL);
			}
			in.close();
			String page = sb.toString();
			return page;

		} catch (ClientProtocolException e) {
			// TODO Auto-generated catch block
		} catch (IOException e) {
			// TODO Auto-generated catch block
		}
		return null;
	}

	public void parseXML(String xmlRecords) {

		DocumentBuilder db = null;
		try {
			db = DocumentBuilderFactory.newInstance().newDocumentBuilder();
		} catch (ParserConfigurationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		InputSource is = new InputSource();
		is.setCharacterStream(new StringReader(xmlRecords));

		Document doc = null;
		try {
			doc = db.parse(is);
		} catch (SAXException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		NodeList nodes = doc.getElementsByTagName("subject");

		for (int i = 0; i < nodes.getLength(); i++) {
			Element element = (Element) nodes.item(i);

			NodeList subj_id = element.getElementsByTagName("subj_id");
			Element line = (Element) subj_id.item(0);
			User u = new User();
			u.subj_id = getCharacterDataFromElement(line);

			NodeList name_subject = element.getElementsByTagName("name_subject");
			line = (Element) name_subject.item(0);
			u.name_subject = getCharacterDataFromElement(line);

			subj.add(u);// add user to list
		}

	}

	public String getCharacterDataFromElement(Element e) {
		Node child = e.getFirstChild();
		if (child instanceof CharacterData) {
			CharacterData cd = (CharacterData) child;
			return cd.getData();
		}
		return "";
	}

	class User {
		public String name_subject;
		public String subj_id;

		@Override
		public String toString() {
			return name_subject;
		}

	}

	private class WebPageTask extends AsyncTask<Void, Void, String> {
		@Override
		protected String doInBackground(Void... param) {

			String xml_text = getData(url, null);
			parseXML(xml_text);
			return null;
			
		}

		@Override
		protected void onPostExecute(String result) {

			Toast.makeText(SelectsubjActivity.this, "Load Succeed...",
					Toast.LENGTH_SHORT).show();
			showAllUser();			
		}
	}

}

<?php
header("Content-type:text/xml");
echo "<?xml version='1.0' encoding='UTF-8'?>";
include("connect.php");

$username = $_POST['username'];

$sql = "SELECT * FROM subject WHERE username = '$username'";
$result = mysql_query($sql,$link);
echo "<data>";
while($row = mysql_fetch_array($result)){
	echo "<subject>";
	echo "<subj_id>".$row['subj_id']."</subj_id>";
	echo "<name_subject>".$row['name_subject']."</name_subject>";
	echo "</subject>";
}
echo "</data>";
?>