What is PHP async programming with ReactPHP or Amp?

Traditional PHP is synchronous and blocking — each request runs in its own process, sequential I/O blocks execution. ReactPHP and Amp are event loop libraries that enable non-blocking I/O in PHP, similar to Node.js. An event loop watches for I/O events (socket ready, timer fired) and dispatches callbacks. ReactPHP uses Promises for async operations. Amp (v3) uses PHP 8.1 Fibers, allowing async code to be written in a synchronous style with $value = await($promise);. Use cases: HTTP servers handling thousands of concurrent connections, WebSocket servers, background job processors, and any I/O-heavy long-running PHP process where spawning a new process per connection is too expensive. Traditional PHP (FPM) is still better for most web apps due to simpler deployment.