Why is PHP unpopular among modern development and why do forums continue to use it

[Sim]

As Chris D mentioned - React (which is a JavaScript framework) and Angular (which is a Typescript framework - ie very similar to JavaScript) are client side languages and are actually complementary to server side languages like PHP.

You'll find a lot of websites out there built with a PHP backend, exposing a JSON API and then they build a JS based front-end which communicates with that PHP backend - they work together.