MainActivity.java

import android.support.v4.app.FragmentManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

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

        FragmentManager fm = getSupportFragmentManager();
        fm.beginTransaction().replace(R.id.escenario, new SesionFragment()).commit();
        }

}

//*******************************************************************
activity_main.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:id="@+id/escenario"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.programming.narvaez.narvalogin.MainActivity">

</android.support.constraint.ConstraintLayout>

//*******************************************************************

Main2Activity.java

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class Main2Activity extends AppCompatActivity {
public static final String nombres="names";
TextView txtBienvenido;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);

        txtBienvenido=(TextView)findViewById(R.id.txtbienvenido);
        String usuario=getIntent().getStringExtra("names");
        txtBienvenido.setText("ЎBienvenido "+ usuario + "!");
    }
}

//*******************************************************************

activity_main2.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.programming.narvaez.narvalogin.Main2Activity">

    <TextView
        android:id="@+id/txtbienvenido"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="TextView"
        android:textColor="@android:color/background_dark"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.117" />
</android.support.constraint.ConstraintLayout>

//******************************************************************
SesionFragment.java


import android.content.Context;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonObjectRequest;
import com.android.volley.toolbox.JsonRequest;
import com.android.volley.toolbox.Volley;

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


public class SesionFragment extends Fragment implements Response.Listener<JSONObject>, Response.ErrorListener {
    RequestQueue rq;
    JsonRequest jrq;
    EditText txtUser, txtPwd;
    Button btnSesion, btnRegistrar;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View vista = inflater.inflate(R.layout.fragment_sesion, container, false);
        txtUser = (EditText) vista.findViewById(R.id.txtuser);
        txtPwd = (EditText) vista.findViewById(R.id.txtpwd);

        btnSesion = (Button) vista.findViewById(R.id.btnsesion);
        btnRegistrar = (Button) vista.findViewById(R.id.btnregistrar);
        rq = Volley.newRequestQueue(getContext());

        btnSesion.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                iniciar_sesion();
            }
        });


        btnRegistrar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                registrar_usuario();
            }
        });


        // Inflate the layout for this fragment
        return vista;
    }

    @Override
    public void onErrorResponse(VolleyError error) {
        Toast.makeText(getContext(), "No Se encontrу el usuario " +error.toString()+ txtUser.getText().toString(), Toast.LENGTH_LONG).show();
    }

    @Override
    public void onResponse(JSONObject response) {
        Toast.makeText(getContext(), "Se encontrу el usuario " + txtUser.getText().toString(), Toast.LENGTH_SHORT).show();
        User usuario = new User();
        JSONArray jsonArray = response.optJSONArray("datos");
        JSONObject jsonObject = null;

        try {
            jsonObject = jsonArray.getJSONObject(0);
            usuario.setUser(jsonObject.optString("user"));
            usuario.setPwd(jsonObject.optString("pwd"));
            usuario.setNames(jsonObject.optString("names"));
        } catch (JSONException e) {
            e.printStackTrace();
        }

        Intent intencion = new Intent(getContext(), Main2Activity.class);
        intencion.putExtra(Main2Activity.nombres, usuario.getNames());
        startActivity(intencion);


    }


    void iniciar_sesion() {
        //192.168.1.66(172.29.243.3
        String url = "http://192.168.1.66/login/sesion.php?user=" + txtUser.getText().toString() +
                "&pwd=" + txtPwd.getText().toString();
        jrq = new JsonObjectRequest(Request.Method.GET, url, null, this, this);
        rq.add(jrq);


    }

    void registrar_usuario(){
        RegistrarFragment fr=new RegistrarFragment();
        //fr.setArguments(fr);
        getActivity().getSupportFragmentManager().beginTransaction()
                .replace(R.id.escenario,fr)
                .addToBackStack(null)
                .commit();

    }


}

//************************************************************
fragment_sesion.xml

<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.programming.narvaez.narvalogin.SesionFragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:id="@+id/textView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:text="@string/hello_blank_fragment"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/user_fragment"
        android:textColor="@android:color/background_dark"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.103"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/textView"
        app:layout_constraintVertical_bias="0.094" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/pwd_fragment"
        android:textColor="@android:color/background_dark"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.03"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView2"
        app:layout_constraintVertical_bias="0.077" />

    <EditText
        android:id="@+id/txtuser"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.275"
        app:layout_constraintStart_toEndOf="@+id/textView2"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.055" />

    <EditText
        android:id="@+id/txtpwd"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.81"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/txtuser"
        app:layout_constraintVertical_bias="0.021" />

    <Button
        android:id="@+id/btnsesion"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="232dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:text="@string/sesion_fragment"
        app:layout_constraintBottom_toBottomOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="@+id/textView"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/btnregistrar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/registrar_fragment"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnsesion"
        app:layout_constraintVertical_bias="0.315" />

</android.support.constraint.ConstraintLayout>

//**********************************************************
RegistrarFragment.java

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonObjectRequest;
import com.android.volley.toolbox.JsonRequest;
import com.android.volley.toolbox.Volley;

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


public class RegistrarFragment extends Fragment implements Response.Listener<JSONObject>, Response.ErrorListener {
    RequestQueue rq;
    JsonRequest jrq;
    EditText txtUser, txtPwd, txtNames;
    Button btnSesion, btnRegistrar;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View vista = inflater.inflate(R.layout.fragment_registrar, container, false);
        txtUser = (EditText) vista.findViewById(R.id.txtuser);
        txtPwd = (EditText) vista.findViewById(R.id.txtpwd);
        txtNames= (EditText) vista.findViewById(R.id.txtnames);

        btnSesion = (Button) vista.findViewById(R.id.btnsesion);
        btnRegistrar = (Button) vista.findViewById(R.id.btnregistrar);
        rq = Volley.newRequestQueue(getContext());

        btnSesion.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                iniciar_sesion();
            }
        });


        btnRegistrar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                registrar_usuario();
            }
        });


        // Inflate the layout for this fragment
        return vista;
    }

    @Override
    public void onErrorResponse(VolleyError error) {
        Toast.makeText(getContext(), "No Se pudу registrar el usuario " +error.toString()+ txtUser.getText().toString(), Toast.LENGTH_LONG).show();
    }

    @Override
    public void onResponse(JSONObject response) {
        Toast.makeText(getContext(), "Se ha registrado el usuario " + txtUser.getText().toString(), Toast.LENGTH_SHORT).show();
         limpiarCajas();
    }

    void limpiarCajas() {
        txtNames.setText("");
        txtUser.setText("");
        txtPwd.setText("");
    }
    void iniciar_sesion() {

        SesionFragment fr=new SesionFragment();
        //fr.setArguments(bn);
        getActivity().getSupportFragmentManager().beginTransaction()
                .replace(R.id.escenario,fr)
                .addToBackStack(null)
                .commit();
    }

    void registrar_usuario(){
        //192.168.1.66(172.29.243.3
        String url = "http://192.168.1.66/login/registrar.php?names=" +txtNames.getText().toString()+"&user="+ txtUser.getText().toString() +
                "&pwd=" + txtPwd.getText().toString();

        jrq = new JsonObjectRequest(Request.Method.GET, url, null, this, this);
        rq.add(jrq);
    }


}

//**********************************************************************

fragment_registrar.xml

<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.programming.narvaez.narvalogin.SesionFragment">

    <!-- TODO: Update blank fragment layout -->

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/user_fragment"
        android:textColor="@android:color/background_dark"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.153"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.267" />

    <EditText
        android:id="@+id/txtuser"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.947"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.236" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/pwd_fragment"
        android:textColor="@android:color/background_dark"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.123"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.416" />

    <EditText
        android:id="@+id/txtpwd"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.407" />

    <Button
        android:id="@+id/btnsesion"
        android:layout_width="123dp"
        android:layout_height="50dp"
        android:layout_marginBottom="8dp"
        android:layout_marginStart="128dp"
        android:layout_marginTop="8dp"
        android:text="@string/sesion_fragment"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.721" />

    <Button
        android:id="@+id/btnregistrar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/registrar_fragment"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.502"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.577" />

    <TextView
        android:id="@+id/textView4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="24dp"
        android:layout_marginLeft="16dp"
        android:layout_marginTop="8dp"
        android:text="@string/names_fragment"
        android:textColor="@android:color/background_dark"
        android:textSize="22sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toStartOf="@+id/txtnames"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.131"
        tools:text="@string/names_fragment" />

    <EditText
        android:id="@+id/txtnames"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="16dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.947"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.1" />

    <TextView
        android:id="@+id/textView5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/registrar_fragment"
        android:textColor="@color/colorPrimary"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.016" />

</android.support.constraint.ConstraintLayout>

//**********************************************************
Clase User.java

/**
 * Created by NARVAEZ on 17/05/2018.
 */

public class User {
    private String names, user, pwd;

    public String getNames() {
        return names;
    }

    public void setNames(String names) {
        this.names = names;
    }

    public String getUser() {
        return user;
    }

    public void setUser(String user) {
        this.user = user;
    }

    public String getPwd() {
        return pwd;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }
}

//*************************************************************
strings.xml

<resources>
<string name="app_name">Narvбez Sesiуn</string>

<!-- TODO: Remove or change this placeholder text -->
<string name="hello_blank_fragment">Inicio de Sesiуn</string>
<string name="user_fragment">Usuario:</string>
<string name="pwd_fragment">Contraseсa:</string>
<string name="sesion_fragment">Inciar Sesiуn</string>
<string name="registrar_fragment">Registrar usuario</string>
<string name="names_fragment">Nombres: </string>

</resources>

//****************************************************************
Dependencias y permisos

Dependencia volley(va en al Module:app - build.gradle)
 compile 'com.android.volley:volley:1.1.0'

//esto se requiere para evitar conflictos de dependencias en anotaciones
configurations.all {
    resolutionStrategy.force 'com.android.support:support-annotations:23.1.0'
}

//permiso de internet, va en el Manifest
<uses-permission  android:name="android.permission.INTERNET"/>

//********************************************************************
Web Service - archivo registrar.php (recordar copiarlo en la ruta:C:xampphtdocslogin у en el path que le generй su servidor cuando lo suba a Internet - ver mi primer vнdeo al respecto:https://youtu.be/EzRZVsw70Rk)

<?PHP
$hostname="localhost";
$database="tu_bd";
$username="root";
$password="";
$json=array();
    if(isset($_GET["names"])&&($_GET["user"]) && isset($_GET["pwd"])){
        $names=$_GET['names']
        $user=$_GET['user'];
        $pwd=$_GET['pwd'];

        $conexion=mysqli_connect($hostname,$username,$password,$database);

        $consulta="INSERT INTO usuarios(names, user, pwd) VALUES ('{$names}','{$user}' , '{$pwd}')";
        $resultado=mysqli_query($conexion,$consulta);


        if($consulta){
           $consulta="SELECT * FROM usuarios  WHERE names='{$names}'";
           $resultado=mysqli_query($conexion,$consulta);

            if($reg=mysqli_fetch_array($resultado)){
                $json['datos'][]=$reg;
            }
            mysqli_close($conexion);
            echo json_encode($json);
        }



        else{
            $results["names"]='';
            $results["user"]='';
            $results["pwd"]='';
            $json['datos'][]=$results;
            echo json_encode($json);
        }

    }
    else{
            $results["names"]='';
            $results["user"]='';
            $results["pwd"]='';
            $json['datos'][]=$results;
            echo json_encode($json);
        }
?>