Swoole is an high-performance network framework using an event-driven, asynchronous, non-blocking I/O model which makes it scalable and efficient. It is written in C language without 3rd party libraries as PHP extension.
It enables PHP developers to write high-performance, scalable, concurrent TCP, UDP, Unix Socket, HTTP, WebSocket services in PHP programming language without too much knowledge about non-blocking I/O programming and low-level Linux kernel.
Compared with other async programming frameworks or softwares such as Nginx, Tornado, Node.js, Swoole has the built-in async, multiple threads I/O modules. Developers can use sync or async API to write the applications.
Will be checking this one out, as an alternative to ReactPHP, since this one comes as a PECL extension. Speed should be heavily better. A quick search around tells me it performs quite well indeed.