This is very poor man's approach to async execution.
Better relying on message queues to process something asynchronously.
(PHP 5 >= 5.3.3, PHP 7, PHP 8)
fastcgi_finish_request — 冲刷(flush)所有响应的数据给客户端
此函数冲刷(flush)所有响应的数据给客户端并结束请求。 这使得客户端结束连接后，需要大量时间运行的任务能够继续运行。
成功时返回 true， 或者在失败时返回 false。
This is very poor man's approach to async execution.
Better relying on message queues to process something asynchronously.
There are some pitfalls you should be aware of when using this function.
The script will still occupy a FPM process after fastcgi_finish_request(). So using it excessively for long running tasks may occupy all your FPM threads up to pm.max_children. This will lead to gateway errors on the webserver.
Another important thing is session handling. Sessions are locked as long as they're active (see the documentation for session_write_close()). This means subsequent requests will block until the session is closed.
You should therefore call session_write_close() as soon as possible (even before fastcgi_finish_request()) to allow subsequent requests and a good user experience.
This also applies for all other locking techniques as flock or database locks for example. As long as a lock is active subsequent requests might bock.