Traditional PHP is synchronous and blocking per request, but libraries like ReactPHP, Amp, and runtimes like Swoole/OpenSwoole provide event loops enabling non-blocking I/O, concurrent HTTP clients, and long-running servers within a single PHP process. Promises or coroutines (often built atop Fibers) let async code read in a mostly linear style.
These approaches are especially valuable for high-concurrency services like WebSocket servers, real-time APIs, or apps that need to fan out many simultaneous outbound HTTP/RPC calls efficiently instead of spawning many blocking processes.