9000 PHP-FPM/SonarQube TCP

What it's for

Port 9000 has two common, unrelated identities depending on the stack you're working in. In PHP environments, it's the conventional port for PHP-FPM (FastCGI Process Manager) to communicate with a web server like Nginx, which proxies incoming HTTP requests to PHP-FPM for actual script execution. Separately, it's also the default web UI port for SonarQube, the popular static code analysis and code-quality platform used in CI pipelines.

Protocol basics

PHP-FPM on port 9000 speaks FastCGI, a protocol specifically designed for a web server to hand off dynamic request processing to an application runtime, rather than plain HTTP — this is why you'll see Nginx configuration referencing fastcgi_pass to port 9000 rather than a normal HTTP proxy directive. SonarQube's use of the same port number is coincidental and purely conventional — it serves a standard HTTP web interface, with no relationship to the PHP-FPM use case beyond sharing a commonly chosen port number.