COPY TO File: Using dynamic file name in stored procedure

Hi,
What would it be the correct format for using a variable in a stored
procedure that uses COPY TO?

I have the current stored procedure:
CREATE FUNCTION Table_To_File(text) RETURNS void AS $delimeter$
COPY (SELECT * FROM table_to_xml('table', true, false, '')) TO '$1'
WITH CSV QUOTE ' ';
$delimeter$
LANGUAGE SQL;

When I run the stored procedure: psql -d <db name> -c "select
Table_To_File('some_absolute_file_name')"; I get the error that I must
use absolute file names.

When I replace TO '$1' with TO '/tmp/toto.xml', the stored procedure
runs fine and creates the local file.

I have tried several different ways to escape the $1 variable to no
avail. At best, I get the same error.