Ok, in this question I learned how to prevent the insert of a NULL value. But, unfortunately, an empty string is being inserted anyway. Apart from preventing this on the PHP side, I'd like to use something like a database constraint to prevent this. Of course a check on the application side is necessary, but I'd like it to be on both sides.

I am taught that whatever application is talking to your database, it should not be able to insert basically wrong data in it. So...

CREATE TABLE IF NOT EXISTS tblFoo (
  foo_id int(11) NOT NULL AUTO_INCREMENT,
  foo_test varchar(50) NOT NULL,
  PRIMARY KEY (foo_id)
);

Would still allow me to do this insert:

INSERT INTO tblFoo (foo_test) VALUES ('');

Which I would like to prevent.