How to integrate Reactjs frontend with php codeigniter application on apache server?

The PHP view files do not need to be replaced with js files. JavaScript can easily be added to PHP files using <script> tags. Below is the Add React in One Minute demo in a CodeIgniter app.

To integrate the React demo into CodeIgniter start with a simple controller - React.php

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class React extends CI_Controller
{
    public function index()
    {
        $this->load->view('react_view');
    }
}

The "view" file is straight from the React demo but it's put in a .php file instead of .html.

The only change made to the demo code is in the script tag at the bottom of the page. My assets folder is on the same level as CodeIgniter's /application folder. There are subfolders in assets for css, js, and images.

/public_html
    /application
    /system
    /assets
        /js
        /css
        /img

So I've changed the src for the tag that loads like_button.js to work with my file layout.

The "view" file react_view.php

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <title>Add React in One Minute</title>
  </head>
  <body>

    <h2>Add React in One Minute</h2>
    <p>This page demonstrates using React with no build tooling.</p>
    <p>React is loaded as a script tag.</p>

    <p>
      This is the first comment.
      <!-- We will put our React component inside this div. -->
      <div class="like_button_container" data-commentid="1"></div>
    </p>

    <p>
      This is the second comment.
      <!-- We will put our React component inside this div. -->
      <div class="like_button_container" data-commentid="2"></div>
    </p>

    <p>
      This is the third comment.
      <!-- We will put our React component inside this div. -->
      <div class="like_button_container" data-commentid="3"></div>
    </p>

    <!-- Load React. -->
    <!-- Note: when deploying, replace "development.js" with "production.min.js". -->
    <script src="https://unpkg.com/react@16/umd/react.development.js" crossorigin></script>
    <script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js" crossorigin></script>

    <!-- Load our React component. -->
    <script src="assets/js/like_button.js"></script>

  </body>
</html>

/assets/js/like_button.js

'use strict';

const e = React.createElement;

class LikeButton extends React.Component {
  constructor(props) {
    super(props);
    this.state = { liked: false };
  }

  render() {
    if (this.state.liked) {
      return 'You liked comment number ' + this.props.commentID;
    }

    return e(
      'button',
      { onClick: () => this.setState({ liked: true }) },
      'Like'
    );
  }
}

// Find all DOM containers, and render Like buttons into them.
document.querySelectorAll('.like_button_container')
  .forEach(domContainer => {
    // Read the comment ID from a data-* attribute.
    const commentID = parseInt(domContainer.dataset.commentid, 10);
    ReactDOM.render(
      e(LikeButton, { commentID: commentID }),
      domContainer
    );
  });