How to combine wordpress_logged_in cookies in one cookie?

Question

I'm trying to optimize my server to consume less resources as possible and increase overall performance. In my site I don't see a need to recompute same requests for users or guests every time. The site is powered by nginx, I enabled fastcgi_cache rules:

fastcgi_cache_path /etc/nginx/cache levels=1:2 keys_zone=MYAPP:250m inactive=60m;
fastcgi_cache_key "$scheme$request_method$host$request_uri";

location ~ \.php$ {
...
                fastcgi_cache MYAPP;

                fastcgi_cache_valid  200 301 302 15m;
                fastcgi_cache_use_stale error timeout updating invalid_header http_500 http_503;
                fastcgi_cache_min_uses 1;
                fastcgi_cache_lock on;
                fastcgi_cache_bypass $skip_cache;
                fastcgi_no_cache $skip_cache;
                add_header X-FastCGI-Cache $upstream_cache_status;
        }

I need to also efficiently cache dynamic and static assets for logged in users too, to optimize my systems further. The idea is to combine ever wordpress_logged_in cookies in singleton cookie, for example cookie_memberships. And pass that HTTP parameter in fastcgi_cache_key “$scheme$request_method$host$request_uri$cookie_memberships”; rule. How can I do that with PHP?