On the main PHP.net site, they've posted the announcement of the release of the latest Release Candidate for the upcoming PHP 7.3.0 version: PHP 7.3.0.RC2.

The PHP team is glad to announce the next PHP 7.3.0 pre-release, PHP 7.3.0RC2. The rough outline of the PHP 7.3 release cycle is specified in the PHP Wiki.

[...] For more information on the new features and other changes, you can read the NEWS file, or the 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.

As with other preview releases you can download this version from the QA site for source packages and the Windows QA site for the Windows binaries.