Help with quote escaping in plpgsql

Hi,

I want to make a function that accepts a pre-formatted varchar
argument which will then be used in an IN clause:

create or replace function testing(varchar) returns int as '
declare
   int c := 0;
begin
   select count(*) from my_table where some_field in ( $1 ) into c;
   return c;
end
' language 'plpgsql';


But I can't figure out how to escape the varchar string I pass. I have
tried:

'''hello'',''world'''

and all sorts of other things.

Any suggestions?

Thanks,

Joseph