A complete example of android with mySQL database.

1.BackgroundWorker.java

package com.example.sharifulislam.potest;

import android.app.AlertDialog;
import android.content.Context;
import android.os.AsyncTask;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLEncoder;

import javax.net.ssl.HttpsURLConnection;

/**
 * Created by Shariful Islam on 02/03/2018.
 */

public class BackgroundWorker extends AsyncTask<String,Void,String> {
    Context context;
    AlertDialog alertDialog;

    BackgroundWorker(Context context){
        this.context = context;
    }

    @Override
    protected String doInBackground(String... voids) {
        String type = voids[0];
        String login_url = "https://your_domain_name/login.php";
        String register_url = "https://your_domain_name/register.php";
        String update_url = "http://your_domain_name/update_data.php";
        String deleted_url = "http://your_domain_name/delete_data.php";
        if (type.equals("login")){
            try {
                String username = voids[1];
                String password = voids[2];
                URL url = new URL(login_url);
                // login url link is "http" link
                HttpsURLConnection httpsURLConnection = (HttpsURLConnection) url.openConnection();
                httpsURLConnection.setRequestMethod("POST");
                httpsURLConnection.setDoOutput(true);
                httpsURLConnection.setDoInput(true);

                OutputStream outputStream = httpsURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String post_data = URLEncoder.encode("username","UTF-8")+"="+URLEncoder.encode(username,"UTF-8")+"&"
                    +URLEncoder.encode("password","UTF-8")+"="+URLEncoder.encode(password,"UTF-8");

                bufferedWriter.write(post_data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                InputStream inputStream = httpsURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String result = "", line = "";
                while ((line = bufferedReader.readLine()) != null){
                    result += line;
                }
                bufferedReader.close();
                inputStream.close();
                httpsURLConnection.disconnect();

                return result;
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        else if (type.equals("register")){
            try {
                String name = voids[1];
                String surname = voids[2];
                String age = voids[3];
                String username = voids[4];
                String password = voids[5];
                URL url = new URL(register_url);
                // register url link is "https" link
                HttpsURLConnection httpsURLConnection = (HttpsURLConnection) url.openConnection();
                httpsURLConnection.setRequestMethod("POST");
                httpsURLConnection.setDoOutput(true);
                httpsURLConnection.setDoInput(true);

                OutputStream outputStream = httpsURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String post_data = URLEncoder.encode("name","UTF-8")+"="+URLEncoder.encode(name,"UTF-8")+"&"
                    +URLEncoder.encode("surname","UTF-8")+"="+URLEncoder.encode(surname,"UTF-8")+"&"
                    +URLEncoder.encode("age","UTF-8")+"="+URLEncoder.encode(age,"UTF-8")+"&"
                    +URLEncoder.encode("username","UTF-8")+"="+URLEncoder.encode(username,"UTF-8")+"&"
                    +URLEncoder.encode("password","UTF-8")+"="+URLEncoder.encode(password,"UTF-8");

                bufferedWriter.write(post_data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                InputStream inputStream = httpsURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String result = "", line = "";
                while ((line = bufferedReader.readLine()) != null){
                    result += line;
                }
                bufferedReader.close();
                inputStream.close();
                httpsURLConnection.disconnect();

                return result;
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        else if (type.equals("update")){
            try {
                String id = voids[1];
                String name = voids[2];
                String surname = voids[3];
                String age = voids[4];
                String username = voids[5];
                String password = voids[6];
                URL url = new URL(update_url);
                // update url link is "http" link
                HttpURLConnection httpsURLConnection = (HttpURLConnection) url.openConnection();
                httpsURLConnection.setRequestMethod("POST");
                httpsURLConnection.setDoOutput(true);
                httpsURLConnection.setDoInput(true);

                OutputStream outputStream = httpsURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String post_data = URLEncoder.encode("id","UTF-8")+"="+URLEncoder.encode(id,"UTF-8")+"&"
                        +URLEncoder.encode("name","UTF-8")+"="+URLEncoder.encode(name,"UTF-8")+"&"
                        +URLEncoder.encode("surname","UTF-8")+"="+URLEncoder.encode(surname,"UTF-8")+"&"
                        +URLEncoder.encode("age","UTF-8")+"="+URLEncoder.encode(age,"UTF-8")+"&"
                        +URLEncoder.encode("username","UTF-8")+"="+URLEncoder.encode(username,"UTF-8")+"&"
                        +URLEncoder.encode("password","UTF-8")+"="+URLEncoder.encode(password,"UTF-8");

                bufferedWriter.write(post_data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                InputStream inputStream = httpsURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String result = "", line = "";
                while ((line = bufferedReader.readLine()) != null){
                    result += line;
                }
                bufferedReader.close();
                inputStream.close();
                httpsURLConnection.disconnect();

                return result;
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        else if (type.equals("delete")){
            try {
                String id = voids[1];
                URL url = new URL(deleted_url);
                // delete url link is "http" link
                HttpURLConnection httpsURLConnection = (HttpURLConnection) url.openConnection();
                httpsURLConnection.setRequestMethod("POST");
                httpsURLConnection.setDoOutput(true);
                httpsURLConnection.setDoInput(true);

                OutputStream outputStream = httpsURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String post_data = URLEncoder.encode("id","UTF-8")+"="+URLEncoder.encode(id,"UTF-8");

                bufferedWriter.write(post_data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();

                InputStream inputStream = httpsURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String result = "", line = "";
                while ((line = bufferedReader.readLine()) != null){
                    result += line;
                }
                bufferedReader.close();
                inputStream.close();
                httpsURLConnection.disconnect();

                return result;
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    @Override
    protected void onPreExecute() {
        alertDialog = new AlertDialog.Builder(context).create();
        alertDialog.setTitle("Server Alert");
    }

    @Override
    protected void onPostExecute(String result) {
        alertDialog.setMessage(result);
        alertDialog.show();
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }
}

2.MainActivity.java

package com.example.sharifulislam.potest;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends Activity{

    EditText user_name, pass;

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

        user_name = (EditText)findViewById(R.id.user_name);
        pass = (EditText)findViewById(R.id.pass);
    }

    public void login_user(View view){
        String userName = user_name.getText().toString();
        String userPass = pass.getText().toString();
        String type = "login";

        BackgroundWorker backgroundWorker = new BackgroundWorker(this);
        backgroundWorker.execute(type, userName,userPass);
    }

    public void go_to_register(View view){
        startActivity(new Intent(MainActivity.this, Register.class));
    }

    public void show_data(View view){
        startActivity(new Intent(MainActivity.this, Show_Data.class));
    }

    public void update_data(View view){
        startActivity(new Intent(MainActivity.this, Update_Data.class));
    }

    public void delete_data(View view){
        startActivity(new Intent(MainActivity.this, Delete_Data.class));
    }
}

3.activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.sharifulislam.potest.MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:padding="10dp">
        <EditText
            android:id="@+id/user_name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/username"/>
        <EditText
            android:id="@+id/pass"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/password"/>
        <Button
            android:onClick="login_user"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/login"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>

        <Button
            android:onClick="go_to_register"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/register"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>

        <Button
            android:onClick="show_data"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/show_data"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>

        <Button
            android:onClick="update_data"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/updaet"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>

        <Button
            android:onClick="delete_data"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/delete"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>
    </LinearLayout>

</RelativeLayout>

4.Register.java

package com.example.sharifulislam.potest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class Register extends AppCompatActivity {

    EditText name, surname, age, user_name, pass;

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

        // cast area
        name = (EditText)findViewById(R.id.name);
        surname = (EditText)findViewById(R.id.surname);
        age = (EditText)findViewById(R.id.age);
        user_name = (EditText)findViewById(R.id.user_name);
        pass = (EditText)findViewById(R.id.pass);

    }

    public void register_user(View view){
        String name_t = name.getText().toString();
        String surname_t = surname.getText().toString();
        String age_t = age.getText().toString();
        String userName = user_name.getText().toString();
        String userPass = pass.getText().toString();
        String type = "register";

        BackgroundWorker backgroundWorker = new BackgroundWorker(this);
        backgroundWorker.execute(type, name_t, surname_t, age_t, userName, userPass);
    }
}

5.activity_register.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.sharifulislam.potest.Register">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:padding="10dp">
        <EditText
            android:id="@+id/name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/name"/>
        <EditText
            android:id="@+id/surname"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/surname"/>
        <EditText
            android:id="@+id/age"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/age"/>
        <EditText
            android:id="@+id/user_name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/username"/>
        <EditText
        android:id="@+id/pass"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/password"/>
        <Button
            android:onClick="register_user"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/register"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>
    </LinearLayout>
</android.support.constraint.ConstraintLayout>

6.Update_Data.java

package com.example.sharifulislam.potest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class Update_Data extends AppCompatActivity {

    EditText id, name, surname, age, user_name, pass;

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

        // cast area
        id = (EditText)findViewById(R.id.id);
        name = (EditText)findViewById(R.id.name);
        surname = (EditText)findViewById(R.id.surname);
        age = (EditText)findViewById(R.id.age);
        user_name = (EditText)findViewById(R.id.user_name);
        pass = (EditText)findViewById(R.id.pass);
    }

    public void update_data(View view){
        String id_t = id.getText().toString();
        String name_t = name.getText().toString();
        String surname_t = surname.getText().toString();
        String age_t = age.getText().toString();
        String userName = user_name.getText().toString();
        String userPass = pass.getText().toString();
        String type = "update";

        BackgroundWorker backgroundWorker = new BackgroundWorker(this);
        backgroundWorker.execute(type, id_t, name_t, surname_t, age_t, userName, userPass);
    }
}

7.activity_update__data.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.sharifulislam.potest.Update_Data">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:padding="10dp">
        <EditText
            android:id="@+id/id"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/id"/>
        <EditText
            android:id="@+id/name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/name"/>
        <EditText
            android:id="@+id/surname"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/surname"/>
        <EditText
            android:id="@+id/age"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/age"/>
        <EditText
            android:id="@+id/user_name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/username"/>
        <EditText
            android:id="@+id/pass"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="@string/password"/>
        <Button
            android:onClick="update_data"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/updaet"
            android:textColor="@color/colorPrimary"
            android:textSize="20sp"/>
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

8.Delete_Data.java

package com.example.sharifulislam.potest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class Delete_Data extends AppCompatActivity {

    EditText data_id;

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

        data_id = (EditText)findViewById(R.id.data_id);
    }

    public void delete_data(View view){
        String id_t = data_id.getText().toString();
        String type = "delete";

        BackgroundWorker backgroundWorker = new BackgroundWorker(this);
        backgroundWorker.execute(type, id_t);
    }
}

9.0.activity_delete__data.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.sharifulislam.potest.Delete_Data"
    android:orientation="vertical"
    android:padding="10dp">

    <EditText
        android:id="@+id/data_id"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/password"/>
    <Button
        android:onClick="delete_data"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/delete"
        android:textColor="@color/colorPrimary"
        android:textSize="20sp"/>
</LinearLayout>

9.1.Show_Data.java

package com.example.sharifulislam.potest;

import android.os.StrictMode;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import org.json.JSONArray;
import org.json.JSONObject;

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class Show_Data extends AppCompatActivity {

    String server_link = "http://your_domain_name/get_data.php";
    InputStream inputStream = null;
    String line = null, result = null;

    EditText input_text;
    TextView show_output;

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

        input_text = (EditText)findViewById(R.id.input_text);
        show_output = (TextView)findViewById(R.id.show_output);
    }

    public void ok_done(View view){
        String passing_data = input_text.getText().toString();

        // allow network in main thread
        StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder().permitNetwork().build());

        // retrieve
        String data = get_data(server_link, "name", passing_data, ",");

        show_output.setText(data);
    }

    public String get_data(String link, String column_name, String pass_data, String splite_data_by){
        String get_data[];
        StringBuffer return_string = new StringBuffer();
        try {
            String new_link = link+"?data_from_app="+pass_data;
            URL url = new URL(new_link);
            HttpURLConnection httpsURLConnection = (HttpURLConnection) url.openConnection();
            httpsURLConnection.setRequestMethod("GET");

            inputStream = new BufferedInputStream(httpsURLConnection.getInputStream());

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        // read input stream into a string
        try{
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
            StringBuilder stringBuilder = new StringBuilder();

            while ((line = bufferedReader.readLine()) != null){
                stringBuilder.append(line + "\n");
            }

            inputStream.close();
            result = stringBuilder.toString();
        }
        catch (Exception e) {
            e.printStackTrace();
        }

        // Parse json data
        try{
            JSONArray jsonArray = new JSONArray(result);
            JSONObject jsonObject = null;

            get_data = new String[jsonArray.length()];

            for (int i=0; i<jsonArray.length(); i++){
                jsonObject = jsonArray.getJSONObject(i);
                get_data[i] = jsonObject.getString(column_name); // column name
            }

            for (int j=0; j<get_data.length; j++){
                if (j==0){
                    return_string.append(get_data[j]);
                }
                else {
                    return_string.append(get_data[j]+splite_data_by);
                }
            }
        }
        catch (Exception e){
            e.printStackTrace();
        }
        return return_string.toString();
    }
}

9.2.activity_show__data.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.sharifulislam.potest.Show_Data"
    android:orientation="vertical">

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/input_text"/>
    <Button
        android:onClick="ok_done"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/show_data"/>

    <TextView
        android:id="@+id/show_output"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/app_name"
        android:textSize="20sp"/>
</LinearLayout>

9.3.AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.sharifulislam.potest">

    <uses-permission android:name="android.permission.INTERNET" />
    "
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".Register"
            android:parentActivityName=".MainActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".MainActivity" />
        </activity>
        <activity
            android:name=".Show_Data"
            android:parentActivityName=".MainActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".MainActivity" />
        </activity>
        <activity
            android:name=".Update_Data"
            android:parentActivityName=".MainActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".MainActivity" />
        </activity>
        <activity android:name=".Delete_Data"
            android:parentActivityName=".MainActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".MainActivity" />
        </activity>
    </application>

</manifest>

9.4.string.xml

<resources>
    <string name="app_name">PO Test</string>
    <string name="username">username</string>
    <string name="password">password</string>
    <string name="name">Name</string>
    <string name="surname">Surname</string>
    <string name="age">Age</string>
    <string name="login">Login</string>
    <string name="register">Register</string>
    <string name="show_data">Show Data</string>
    <string name="updaet">Update</string>
    <string name="id">ID</string>
    <string name="delete">Delete</string>
</resources>

9.5.conn.php

<?php 
$db_name = "database_name";
$mysql_username = "mysql_username";
$mysql_password = "mysql_password";
$server_name = "localhost";

$conn = mysqli_connect($server_name, $mysql_username, $mysql_password, $db_name);

/*
if ($conn) {
	echo "connection success!";
}
else{
	echo "connection not success!";
}
*/
 ?>

9.6.login.php

<?php 
require "conn.php";
$user_naem = $_POST["username"];
$user_pass = $_POST["password"];

$mysql_qry = "select * from android_data where username like '$user_naem' and password like '$user_pass';";

$result = mysqli_query($conn, $mysql_qry);

if (mysqli_num_rows($result)>0) {
$row = mysqli_fetch_assoc($result);
$name = $row["username"];

	echo "login success! Welcome " .$name;
}
else{
	echo "login faile!";
}

 ?>

9.7.register.php

<?php 
require "conn.php";

$name = $_POST["name"];
$surname = $_POST["surname"];
$age = $_POST["age"];
$username = $_POST["username"];
$password = $_POST["password"];

$mysql_qry = "insert into android_data (name, surname, age, username, password) values ('$name', '$surname', '$age', '$username', '$password')";

if ($conn->query($mysql_qry) === TRUE) {
	echo "Insert Successful";
}
else{
	echo "Error : " . $mysql_qry . "<br>" . $conn->error;
}

$conn->close();

 ?>

9.8.update_data.php

<?php 
require "conn.php";

$id = $_POST["id"];
$name = $_POST["name"];
$surname = $_POST["surname"];
$age = $_POST["age"];
$username = $_POST["username"];
$password = $_POST["password"];

// Select Database
mysqli_select_db($conn, "id4931910_android1");

// update query
$sql = "update android_data set name='$name', surname='$surname',age='$age', username='$username', password='$password' where id='$id'";

// Execuet query
if (mysqli_query($conn,$sql)) {
	echo "update successfully!";
}
else{
	echo "not update data.";
}


 ?>

9.9.0.delete_data.php

<?php 
require "conn.php";

$id = $_POST["id"];

// Select Database
mysqli_select_db($conn, "id4931910_android1");

// update query
$sql = "delete from android_data where id='$id'";

// Execuet query
if (mysqli_query($conn,$sql)) {
	echo "delete successfully!";
}
else{
	echo "not delete data.";
}


 ?>

9.9.1.get_data.php

<?php 
require "conn.php";

$user_name = $_GET["data_from_app"];

$query = mysqli_query($conn, "select * from android_data where username='$user_name'");

if ($query) {
	while ($row = mysqli_fetch_array($query)) {
		$flag[] = $row;
	}
	print(json_encode($flag));
}

$conn->close();
 ?>