how are concurrent requests handled in PHP (using - threads, thread pool or child processes)

Question

I understand that PHP supports handling multiple concurrent connections and depending on server it can be configured as mentioned in this answer

How does server manages

Answer 1

After doing some research I ended up with below conclusions.

It is important to consider how PHP servers are set to be able to get insights into it.For setting up the server and PHP on your own, there could be three possibilities:

1) Using PHP as module (For many servers PHP has a direct module interface (also called SAPI))

2) CGI

3) FastCGI

Considering Case#1 PHP as module, in this case the module is integrated with the web server itself and now it puts the ball entirely on web server how it handles requests in terms of forking process, using threads, thread pools, etc.

For module, Apache mod_php appears to be very commonly used, and the Apache itself handles the requests using processes and threads in two models as mentioned in this answer

Prefork MPM uses multiple child processes with one thread each and each process handles one connection at a time.

Worker MPM uses multiple child processes with many threads each. Each thread handles one connection at a time.

Obviously, other servers may take other approaches but, I am not aware of same.

For #2 and #3, web server and PHP part are handled in different processes, and how a web server handles the request and how it is further processed by application(PHP part) varies. For e.g.: NGINX may handle the request using asynchronous non-blocking I/O and Apache may handle requests using threads, but, how the request would be processed by FastCGI or CGI application is a different aspect as described below. Both the aspects i.e. how web server handles requests and how PHP part is processed would be important for PHP servers performance.

Considering #2, CGI protocol has makes web server and application (PHP) independent of each other and CGI Protocol requires application and web server to be handled using different process and the protocol does not promote reuse of the same process, which in turn means a new process is required to handle each request.

Considering#3, FastCGI protocol overcomes the limitation of CGI by allowing process re-use. If you check IIS FastCGI link FastCGI addresses the performance issues that are inherent in CGI by providing a mechanism to reuse a single process over and over again for many requests.

FastCGI maintains compatibility with non-thread-safe libraries by providing a pool of reusable processes and ensuring that each process handles only one request at a time.

That said, in case of FastCGI it appears that the server maintains a process pool and it uses the process pool to handle incoming client requests and since, the process pool does not require thread safe check, it provides a good performance.