I don't know enough about PHP to understand this comment. Can you elaborate?
You could make asynchronous servers using socket_select(), which was added in PHP 4.1.
https://www.php.net/manual/en/function.socket-select.php
There was also a third party extension that added threads to PHP 15 years ago, although it was buggy.
Reminds me of ReactPHP (no, not that React) that allowed you to do non-blocking I/O in PHP back in like 2012-2013 sometime. Maybe that's the library you're referring to actually?
we have a linter that bitches anytime anyone uses php's empty. In fact, php has tons of these weird behaviours mostly masaradng as trying to be helpful
Seems they still have the docs up: https://www.php.net/manual/en/book.pthreads.php