What is PHP Fibers?

Fibers (PHP 8.1) are units of concurrency that allow functions to be paused and resumed. They are PHP's implementation of coroutines at the language level. A Fiber can be paused mid-execution with Fiber::suspend($value) and resumed with $fiber->resume($value). Unlike threads, Fibers are cooperative — they yield control explicitly and run in a single thread. They enable non-blocking I/O patterns and are the foundation for asynchronous PHP frameworks like ReactPHP and Amp (v3+). Example: $fiber = new Fiber(function(): void { $value = Fiber::suspend("paused"); }); $result = $fiber->start(); $fiber->resume("hello");. Fibers are lower-level than async/await but enable async libraries to be built on top of them.