[...] For more information on the new features and other changes, you can read the NEWS file, or the <a href="https://github.com/php/php-src/blob/php-7.3.0beta2/UPGRADING>UPGRADING file for a complete list of upgrading notes. Internal changes are listed in the UPGRADING.INTERNALS file. These files can also be found in the release archive.
You can download this latest preview release from either the source package site or from the Windows QA site for the Windows binaries.