PL/PGSQL - character varying as function argument

I'm fairly new at PL/PGSQL and I'm trying to create a login function.
I want to pass the username and password to the function and return
the permission type that user has. Here's a shortened version of the
function with just the part giving me problems.

CREATE OR REPLACE FUNCTION user_checkCredentials(character varying)
RETURNS character varying AS
$BODY$
DECLARE
username ALIAS FOR $1;
permission record;
BEGIN
select into permission permtype from users;
RETURN permission.permtype;
END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE
COST 100;

When I try to run it with:
select user_checkCredentials("asdf");

I get the error:
ERROR: column "asdf" does not exist
LINE 1: select user_checkCredentials("asdf");

However if I use numbers, say an integer, and change to
user_checkCredentials(integer) and pass a number it works. I obviously
don't understand how function arguments work in PL/PGSQL so can
someone explain to me what's going on?