Understanding CGI, FastCGI, and php-fpm in PHP Backend Development

CGI is a protocol independent of processes; when a web server such as Nginx receives a PHP request, it follows its configuration to locate the PHP interpreter, packages request information according to the CGI standard, and passes it to the interpreter.

PHP implements the CGI protocol; after receiving the request, the interpreter reads php.ini and other initialization settings, processes the data, and returns a response formatted as CGI data back to Nginx, which then forwards it to the browser.

FastCGI is a persistent‑process manager that differs from traditional CGI, which spawns a new process for each request. FastCGI starts multiple PHP processes when the web server starts, allowing subsequent requests to be handled by already‑running processes, reducing overhead and improving response speed.

php‑fpm builds on FastCGI by adding features that address several drawbacks of the basic FastCGI setup, such as the need to restart the web server after changing php.ini, the inability to gracefully restart php‑cgi processes, and other management limitations. Using php‑fpm resolves these issues and provides smoother, more efficient PHP execution.