Pub/Sub between PHP server and React client using FCM

I have been trying to set up a messaging system between my PHP server and React front end. I managed to publish messages from the PHP server as can be seen in the Metrics:
FCM API metrics showing API requests from PHP server

The issue I am facing is that I can not receive the messages from React, as per the documentation for the client SDK, there are functions for messaging. But the same page also mentions that I need to use the admin SDK instead??

As for the admin SDK I can not use it since it requires me to upgrade React in my project which can lead to issues.