How to INSERT empty line into SEQUENTIAL table from PHP

Hi,

rahter dump question, I guess....

But I have a table with a sequential index field, into which I would
like to add from time to time another line (via webbrowser), which in
turn, stays first empty, before it's being filled in later (via
webbrowser).

Because the ID field is sequential and indexed, I can't use

INSERT INTO table_xx (ID, field2, field3, field4) VALUES ('', '', '',
'');

neither (skipping ID because it should be filled in automatically):

INSERT INTO table_xx (field2, field3, field4) VALUES ('', '', '');

How am I supposed to do it?

Thanks for any hints,

Stef