PHP 8.1 comes with a few new additions to the language, and one that I have seen get the most attention is fibers. This is a feature that allows PHP code to be executed in a concurrent thread, with the ability to pause and resume that code at will.
Using fibers doesn't mean that the code runs in parallel, rather that the code is executed away from the main thread in a virtual or green thread. These are threads create by and executed by the PHP VM, rather than being executed by the CPU and managed by the underlying OS. This lightweight thread of execution is also called a coroutine and are executed in sequence, rather than being parallel.
Fibers are intended to eliminate the distinction between synchronous and asynchronous functions in PHP and provide a petter mechanism to manage blocking code.