How to set parameters in 'options'?

Hi,

I'd like to write a function like this:

create function foo(_host text, _port text, _dbname text)
returns void as $$
  create server _server
    foreign data wrapper postgres_fdw
    options (host _host, port _port, dbname _dbname);
    -- ERROR: syntax error at or near "_host"
$$ language sql;

In order to e.g. do:
select foo('111.11.11.11', '5432', 'mydb');

How can I set the parameters in 'options' using those variables?

:Stefan

P.S. Actually I'll put this code in a plggsql function later on.