Returning 0 rows from a PL/PGSQL

I have the following plpgsql function:

CREATE OR REPLACE FUNCTION public."temp"(int4)
RETURNS public.books AS
$BODY$DECLARE
old_book books%rowtype;
BEGIN
select * into old_book from books
where book_id = var_book_id;

IF FOUND = false THEN
return null;
ELSE
return old_book;
END IF;
END;$BODY$
LANGUAGE 'plpgsql' VOLATILE;

If the function finds a book with the given ID, it returns its row, if
it doesn't, it should return no rows at all (naturally it is
simplified version of what I need). In practice, however, it returns
either a regular row, or a regular row with all fields set to NULL.

So if in my PHP code I have:

$rs = pg_query("select * from temp(-60)");
pg_num_rows($rs);

It keeps printing 1 even if the ID I pass doesn't exist. What's my remedy?

Thanks

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