Integrate react.js on existing php webpages

index.js

import React from 'react';
import ReactDOM from 'react-dom';

const entries = {
    "#login": () => import(/* webpackChunkName: "login" */ "./login.js"), 
    // ^^^^^ this chunk will be created as "login.js".
    // and then we can add dom element "#login" and import bundle.js and login.js when we want to use `Login` component.
};

Object.keys(entries).some(selector => {
    const rootContainer = document.querySelector(selector);
    if (!rootContainer) return false;

    entries[selector]().then(({ default: Component }) => {
        ReactDOM.render(<Component />, rootContainer);
    });

    return true;
});

login.js

import React, { Component } from 'react';

class Login extends Component {
    render() {
        return (
            <div>
                Hello
            </div>
        );
    }
}

export default Login;