Getting error on react native "json parse error unexpected eof"

I have followed this code from the site "http://carlofontanos.com/user-login-with-wordpress-using-react-native/"

I have done my Login.js like this

import React, { Component } from 'react';
import {
  AppRegistry,
  StyleSheet,
  Text,
  View,
  TextInput,
  TouchableOpacity,
  Keyboard
} from 'react-native';
import { AsyncStorage } from 'react-native';
import {Actions} from 'react-native-router-flux';
import { StackNavigator } from 'react-navigation';
export default class LoginForm extends Component<{}> {

  constructor(props){
    super(props)
    this.state={
      userEmail:'',
      userPassword:'',
      validating: false
    }
  }

  login = () =>{
    this.state.validating = true;
    const {userEmail,userPassword} = this.state;
    let reg = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/ ;
    if(userEmail==""){
      //alert("Please enter Email address");
      this.setState({email:'Please enter Email address'})

    }

    else if(reg.test(userEmail) === false)
    {
    //alert("Email is Not Correct");
    this.setState({email:'Email is Not Correct'})
    return false;
      }

    else if(userPassword==""){
    this.setState({email:'Please enter password'})
    }
    else{

    fetch('http://siteURL/wetest/userlogin.php',{
      method:'post',
      header:{
        'Accept': 'application/json',
        'Content-type': 'application/json'
      },
      body:JSON.stringify({
        email: userEmail,
        password: userPassword
      })

    })
    .then((response) => response.json())
     .then((responseJson)=>{
        let data = responseJson.data;
        if (this.saveToStorage(data)){
          this.setState({
            validating: false
          });
          alert("Successfully Login");
          /* Redirect to home page */
          Actions.home()
        } else {
          alert("Wrong Login Details");
        }

     })
     .catch((error)=>{
     console.error(error);
     });
    }


    Keyboard.dismiss();
  }
    render(){
        return(
            <View style={styles.container}>
          <TextInput style={styles.inputBox} 
              underlineColorAndroid='rgba(0,0,0,0)' 
              placeholder="Email"
              placeholderTextColor = "#ffffff"
              selectionColor="#fff"
              keyboardType="email-address"
              onChangeText={userEmail => this.setState({userEmail})}
              />
          <TextInput style={styles.inputBox} 
              underlineColorAndroid='rgba(0,0,0,0)' 
              placeholder="Password"
              secureTextEntry={true}
              placeholderTextColor = "#ffffff"
              ref={(input) => this.password = input}
              onChangeText={userPassword => this.setState({userPassword})}
              />  
           <TouchableOpacity style={styles.button} onPress={this.login} >
             <Text style={styles.buttonText}>Login</Text>
           </TouchableOpacity>     
        </View>
            )
    }

  async saveToStorage(userData){
    if (userData) {
      await AsyncStorage.setItem('user', JSON.stringify({
          isLoggedIn: true,
          authToken: userData.auth_token,
          id: userData.user_id,
          name: userData.user_login
        })
      );
      return true;
    }

    return false;
  }
}

And i have done the server site code like this.

*

<?php 
    include 'wp-load.php';
    $json = file_get_contents('php://input');   
    $obj = json_decode($json,true);
    $email = $obj['email'];

    $password = $obj['password'];
$response = array(
    'data'      => array(),
    'msg'       => 'Invalid email or password',
    'status'    => false
);  
    if($obj['email']!=""){  
    $creds = array();
    $creds['user_login'] = $email;
    $creds['user_password'] = $password;
    $creds['remember'] = true;

    $user = wp_signon( $creds, false );

        if ( is_wp_error($user) ){
            echo json_encode('Wrong Details');              
        }
        else{
            $user = get_user_by( 'email', $email );     
            if ( $user ){
                $password_check = wp_check_password( $_POST['password'], $user->user_pass, $user->ID );

                if ( $password_check ){
                    /* Generate a unique auth token */
                    $token = wp_generate_password( 30 );

                    /* Store / Update auth token in the database */
                    if( update_user_meta( $user->ID, 'auth_token', $token ) ){

                        /* Return generated token and user ID*/
                        $response['status'] = true;
                        $response['data'] = array(
                            'auth_token'    =>  $token,
                            'user_id'       =>  $user->ID,
                            'user_login'    =>  $user->user_login
                        );
                        $response['msg'] = 'Successfully Authenticated';
                        //echo json_encode($response);  
                    }
                }
            }           
        }
    }   
    else{
      echo json_encode('try again');
    }

?>

*

If you can see the both code it is written for login and save the data in the device by "async saveToStorage". But for now it giving me the error. The error is "json parse error unexpected eof". You can in the php code, I have tried with return the data by json_encode. That did not work also. Can i get some on this. I want to know where the exact error is ? Thanks in advance.