PHP 5.3.0 is only in its first alpha release. But, if you run PHP in Microsoft Windows, you should talk a look at the release notes for Windows binaries.

Among the changes are things like the drop of support for Windows versions less than Win2000, the changeover for compiling needs (VC6 versus the newer VC9) and that 64-bit binaries will be provided as well as the normal 32-bit (on a trial basis).