PHP 5.1 is now in beta, and introduces PHP Data Objects (PDO).

The PDO library lets you access any of an extensive selection of supported databases using a single set of functions. So not only do you only need to learn one set of functions to code to multiple databases, but you can transition an application to another database without having to change all the functions throughout your code.

PHP developers have been achieving this for years using database abstraction layers, at first homegrown, then open source, and finally official. This is the first time such an abstraction layer will be completely built into PHP, and not a layer of PHP code sitting on top of it, however.