I'm having the exact same issue - there doesn't seem to be a socket_auth function in the php file defining the class, and my Google searches have been very unhelpful. One person who had the issue ended up using the facade (from pusher-http-laravel) which I've tried and had the same issue with. In a reply to that question, somebody said the class had a __call function but I can't find them in vendor/pusher/pusher-php-server/src/Pusher.php

Quick update - it looks like composer is installing pusher-php-server from the dev-new-lib branch, which satisfies a version requirement of 3.0 because its semantic version is "3.0.x-dev".

I was able to resolve my problem by running:

composer remove pusher/pusher-php-server
composer require pusher/pusher-php-server:v3.0.1