PHP_FCGI_MAX_IDLE_TIMEOUT environment variabile option for php-cgi

Description

I'm writing an "external" multiplatform php fastcgi process manager in golang for the caddy webserver, something like php-fpm but using the php-cgi executable in fastcgi mode;

while implementing it using socket passing to be faster simpler and have zero-copy between processes

i've found that i wanted to add an option in my process manager analogous to php-fpm pm.process_idle_timeout to free memory when the server load is low and to avoid eventual memory leaks

I've thinked about how to implement this feature for a while in a way that does not require modifying php-cgi itself, but without finding a solution that allowed to keeping zero-copy socket passing.

The proposal is to add an environment variabile option that instruct php-cgi processes to self terminate after a certain amount of time if no requests are received, something like the already existing PHP_FCGI_MAX_REQUESTS that already aligns with php-fpm pm.max_requests.

Eventually having millisecond granularity unlike second granularity only like in pm.process_idle_timeout will be a plus.

That Is... Hoping i'm not missing something obviuos and that the proposal make sense.

Start by reviewing the php-cgi FastCGI process lifecycle and the existing PHP_FCGI_MAX_REQUESTS option, since no source file or test is named. Define how an idle timeout should be configured and measured, then add coverage showing that an idle process terminates while a process receiving requests remains active.