dba_open cannot write

I am trying to write data to a db3 alias file that sendmail uses (made a copy for testing). When I read the file using: $id = dba_open ("aliaseswww.db", "r", "db3"); It works. But when I try to change it to: $id = dba_open ("aliaseswww.db", "w", "db3"); I get: dba_open(aliaseswww.db,w): Driver initialization failed for handler: db3: Invalid argument in /usr/local/www/data/maillists/test.php on line 5 I don't know what the "Invalid argument" is? It seems as if it is missing the "db3" on the end of the dba_open, but I put it there. I have checked file permisions and everything checks out. Any ideas? -Bob