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java - Passing ID with PHP and MySQL

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I am programming an app where a teacher logs in with a username and password. After the login a new fragment in the app opens where the teacher sees all his students in aListView. To login I only type the username and the password. I use for this php and mysql.

My problem is that mylogin.php doesn't pass the ID from the teacher to theget_data.php where I do a SQL query.

login.php

<?php
session_start();
if($_SERVER['REQUEST_METHOD']=='POST'){
    $username = $_POST['username'];
    $password = $_POST['password'];       
    if($username == '' || $password == ''){
        echo '';
    }else{
        require_once('dbConnect.php');
        $sql = "SELECT ID_Teacher, username, password FROM Lehrer WHERE username='$username' and password='$password'";
        mysqli_error($con);
        $check = mysqli_fetch_array(mysqli_query($con,$sql));                    
    }
    if(isset($check)){
        echo "success";
        $_SESSION['id'] = $check['ID_Teacher'];
    }else{
        echo "Wrong Password or Username";
    }
}else{
    echo "Error, try again!";
}
?>

get_data.php

<?php
session_start();
require_once('dbConnect.php');
$sql = "SELECT Student.Name,Student.Surname FROM Student WHERE Student.ID_Teacher = {$_SESSION['id']}";
$res = mysqli_query($con,$sql);
$result = array();
while($row = mysqli_fetch_array($res))  
{  
    array_push($result, array('Name'=>$row[0], 'Surname'=>$row[1]));  
}
echo json_encode(array("result"=>$result));
mysqli_close($con);
?>

ActivityLogin.java

import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import java.util.HashMap;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import java.util.HashMap;

public class ActivityLogin extends AppCompatActivity implements View.OnClickListener{

public static final String USER_NAME = "USER_NAME";

public static final String PASSWORD = "PASSWORD";

private static final String LOGIN_URL = "http://bachelormedinf.16mb.com/login.php";

private EditText editTextUserName;
private EditText editTextPassword;

private Button buttonLogin;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_activity_login);

    editTextUserName = (EditText) findViewById(R.id.username);
    editTextPassword = (EditText) findViewById(R.id.password);

    buttonLogin = (Button) findViewById(R.id.buttonUserLogin);

    buttonLogin.setOnClickListener(this);
}


private void login(){
    String username = editTextUserName.getText().toString().trim();
    String password = editTextPassword.getText().toString().trim();
    userLogin(username,password);
}

private void userLogin(final String username, final String password){
    class UserLoginClass extends AsyncTask<String,Void,String> {
        ProgressDialog loading;
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            loading = ProgressDialog.show(ActivityLogin.this,"Please Wait",null,true,true);
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            loading.dismiss();
            if(s.equalsIgnoreCase("success")){
                Intent intent = new Intent(ActivityLogin.this,ListView.class);
                intent.putExtra(USER_NAME,username);
                startActivity(intent);
            }else{
                Toast.makeText(ActivityLogin.this, s, Toast.LENGTH_LONG).show();
            }
        }

        @Override
        protected String doInBackground(String... params) {
            HashMap<String,String> data = new HashMap<>();
            data.put("username",params[0]);
            data.put("password",params[1]);

            RegisterUserClass ruc = new RegisterUserClass();

            String result = ruc.sendPostRequest(LOGIN_URL,data);

            return result;
        }
    }
    UserLoginClass ulc = new UserLoginClass();
    ulc.execute(username,password);
}

@Override
public void onClick(View v) {
    if(v == buttonLogin){
        login();
    }
}}

ListView.java

import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.text.method.ScrollingMovementMethod;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;


public class ListView extends ActionBarActivity implements View.OnClickListener {

private TextView textViewJSON;
private Button buttonGet;
private Button buttonParse;
private TextView textView;

public static final String MY_JSON ="MY_JSON";

private static final String JSON_URL = "http://www.bachelormedinf.16mb.com/get_data.php";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    textViewJSON = (TextView) findViewById(R.id.textViewJSON);
    textViewJSON.setMovementMethod(new ScrollingMovementMethod());
    buttonGet = (Button) findViewById(R.id.buttonGet);
    buttonParse = (Button) findViewById(R.id.buttonParse);
    buttonGet.setOnClickListener(this);
    buttonParse.setOnClickListener(this);

    textView = (TextView) findViewById(R.id.textViewUserName);

    Intent intent = getIntent();

    String username = intent.getStringExtra(ActivityLogin.USER_NAME);

    textView.setText("Willkommen Dr. " + username);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@Override
public void onClick(View v) {
    if(v==buttonGet){
        getJSON(JSON_URL);
    }

    if(v==buttonParse){
        showParseActivity();
    }
}

private void showParseActivity() {
    Intent intent = new Intent(this, ParseJSON.class);
    intent.putExtra(MY_JSON,textViewJSON.getText().toString());
    startActivity(intent);
}


private void getJSON(String url) {
    class GetJSON extends AsyncTask<String, Void, String>{
        ProgressDialog loading;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            loading = ProgressDialog.show(ListView.this, "Please Wait...",null,true,true);
        }

        @Override
        protected String doInBackground(String... params) {

            String uri = params[0];

            BufferedReader bufferedReader = null;
            try {
                URL url = new URL(uri);
                HttpURLConnection con = (HttpURLConnection) url.openConnection();
                StringBuilder sb = new StringBuilder();

                bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));

                String json;
                while((json = bufferedReader.readLine())!= null){
                        sb.append(json+"\n");
                }

                return sb.toString().trim();

            }catch(Exception e){
                return null;
            }

        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            loading.dismiss();
            textViewJSON.setText(s);
        }
    }
    GetJSON gj = new GetJSON();
    gj.execute(url);
}}

RegisterUserClass.java

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.HashMap;
import java.util.Map;

import javax.net.ssl.HttpsURLConnection;


public class RegisterUserClass {

public String sendPostRequest(String requestURL,
                              HashMap<String, String> postDataParams) {

    URL url;
    String response = "";
    try {
        url = new URL(requestURL);

        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(15000);
        conn.setConnectTimeout(15000);
        conn.setRequestMethod("POST");
        conn.setDoInput(true);
        conn.setDoOutput(true);


        OutputStream os = conn.getOutputStream();
        BufferedWriter writer = new BufferedWriter(
                new OutputStreamWriter(os, "UTF-8"));
        writer.write(getPostDataString(postDataParams));

        writer.flush();
        writer.close();
        os.close();
        int responseCode=conn.getResponseCode();

        if (responseCode == HttpsURLConnection.HTTP_OK) {
            BufferedReader br=new BufferedReader(new InputStreamReader(conn.getInputStream()));

            response = br.readLine();
        }
        else {
            response="Error Registering";
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

    return response;
}

private String getPostDataString(HashMap<String, String> params) throws UnsupportedEncodingException {
    StringBuilder result = new StringBuilder();
    boolean first = true;
    for(Map.Entry<String, String> entry : params.entrySet()){
        if (first)
            first = false;
        else
            result.append("&");

        result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
        result.append("=");
        result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
    }

    return result.toString();
}
}
878

Answer

Solution:

try this

  $sql = "SELECT Student.Name,Student.Surname FROM Student WHERE Student.ID_Teacher = ".$_SESSION['id'];

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