Unfortunately, because curl_exec is a blocking call, your server will actually hang whenever a post is made to the Discord webhook API. I did this a few weeks ago, and even with two users online, the effect on performance was noticeable, and this could very easily be abused by creating bots which login to the server repeatedly.
PHP is able to execute multiple curl requests at once, but there isn't a way (as far as I know), to carry on with your socket server loop before the requests are complete, not without using an extension which brings real multithreading to PHP, like pthreads.
A nice idea, and it works, to a degree, don't use this if your server has many players. :)