http://www.postgresql.org/: PostgreSQL home page
http://www.php.net/: PHP home page
http://www.inquiry.com/techtips/thesqlpro/: ask the SQL pro at inquiry.com, which is geared towards Microsoft products, but has some useful information on standard SQL as well.
photo.net is a large, well-engineered, database-backed web site which also features a number of useful articles and HOWTOs for web developers.
http://photo.net/sql: SQL for Web Nerds, a great reference and an even better introduction.
http://photo.net/wtr/thebook: Philip and Alex's Guide to Web Publishing, a great book-on-the-web, although you'll probably want to buy the print edition for the great photographs.
http://software.arsdigita.com: Arsdigita.com, a consulting firm which makes an open-source toolkit for database-backed web sites. Unfortunately, it relies on AOLserver and Oracle, which are not open source, although AOLserver may be by the time you read this.
Fundamentals of Database Systems, Ramez Elmasri and Shamkant Navathe, ISBN 0805317481. This is a great introduction to database theory and implementation techniques, with a continual emphasis on application.
Tractatus Logico-Philosophicus, Ludwig Wittgenstein, translated by C.K. Ogden, ISBN 041505186X. This is a logician's guide to the concepts underlying object-oriented design and data modeling, and should be required reading in undergraduate computer science classes. This particular edition has the original German text en face.
Code Complete: A Practical Handbook of Software Construction, Steve McConnell, ISBN 1556154844. This is a sort of “Mythical Man-Month” for the microcomputer age—it has many good tips to keep in mind before you start on a big project.