Although you've restricted your PHP/Web server to the UNIX platform, there's nothing to stop your database server from being a completely different physical machine. In fact, many of the more high-traffic sites and applications you build will almost certainly require a separate database server or, in some cases, two or three.
With this in mind, therefore, the doors are flung wide open. You have to choose between PostgreSQL, MySQL, Oracle, Informix, DB2, Microsoft SQL Server, SAP DB, and many more.
Indeed, all the previous suggestions would be excellent choices. They are all highly accomplished and frequently updated. Because database servers are not exposed to the Internet, any security flaws discovered are of little importance. As long as a proper firewall is in place, they are all as secure as each other.
There are minor discrepancies in their notional reliability, but in all honesty, for every champion of, say, SQL Server, there is also a champion of MySQL, Oracle, DB2, and so on.
They're all fast, too. Any real difference in performance often comes down to the way in which you have structured your database rather than having anything to do with the database server itself.
With this in mind, you might struggle to choose. So have we, but in this book we have encouraged the use of PostgreSQL. Why? A number of reasons.
First, we wanted to stick to free software as much as possible, because Apache and PHP are both free of charge to download and use. MySQL isn't, strictly speaking, free. PostgreSQL is, and this means it passes the first test.
Second, PostgreSQL is about as close to industry standard heavyweights such as Oracle as you can get without actually being Oracle. Its syntax is largely ANSI compliant, meaning that porting an application away from PostgreSQL to another database is a relatively straightforward job.
Finally, it's great all around. In addition to being free and standards compliant, it's also extremely fast, extremely stable, and pretty easy to get set up and installed. It's also very well supported by PHP, which is a huge bonus given the title of this book.