How to set vhost-specific ENV vars for pass to php-fpm/fastcgi?

[pgndev]

I run nginx + php-fpm.

I include the following 'secure' config into my shared site config

    cat ../sec/params.shared
        fastcgi_param DATABASE_HOST     "localhost";
        fastcgi_param DATABASE_PORT     "3306";
        fastcgi_param DATABASE_NAME     "db_name";
        fastcgi_param DATABASE_USER     "db_user";
        fastcgi_param DATABASE_PASSWORD "db_pass";

It works as intended, setting the ENV vars in PHP env.

I'd like to add some vhost-specific env vars, that ONLY exist & are set for
specific vhost's ENVs

For example, I'd like to set up a conditional add'n of

        set $this_token ""
        if ($host = 'vhost1.example.com') {
            set $this_token "data";
10            fastcgi_param VHOST1_TOKEN  $this_token;
        }

But when I add this stanza to the main config above, conf check fails

    nginx: [emerg] "fastcgi_param" directive is not allowed here in
/etc/nginx/sec/param.shared:10
    nginx: configuration file /etc/nginx/nginx.conf test failed

If instead I add

        set $this_token ""
        if ($host = 'vhost1.example.com') {
            set $this_token "data";
        }
        fastcgi_param VHOST1_TOKEN  $this_token;

it passes check

    nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
    nginx: configuration file /etc/nginx/nginx.conf test is successful

In this case the ENV var "VHOST1_TOKEN" is corrrectly defined in the match
case, but it is also *defined*, though null, for any/all other vhosts.

How do I construct this conditional ENV var setting to ONLY set/defined the
vars on host match?
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