Description:
------------
When using PHP-FPM, you can configure the "user" directive for a pool with either an (alphanumeric) username or a (numeric) uid. However, if you do the latter, initgroups() won't set supplementary groups.

This is due to fpm_unix_init_child() in sapi/fpm/fpm/fpm_unix.c calling "initgroups(wp->config->user, wp->set_gid)".

One possible solution would be changing this to "initgroups(wp->user, wp->set_gid)" which would require to set wp->user from getpwuid(wp->set_uid) in fpm_unix_conf_wp() in the very same file, which currently is only done when is_root is false.

One objection could be that a uid might not be unique, but the same applies to the non-root case. Another possibility then might be an explicit configuration directive for supplementary groups.

If you decide to go for the first solution, I'd volunteer to write a patch for that upon request.