On the main PHP.net site there's an announcement posted about the release of the latest pre-release of the PHP 7.3.x series: PHP 7.3.0.beta1.

The PHP team is glad to announce the release of the fifth PHP 7.3.0 version, PHP 7.3.0beta1. The rough outline of the PHP 7.3 release cycle is specified in the PHP Wiki.

[...] Please carefully test this version and report any issues found in the bug reporting system.

Changes in this beta include fixes in the language core, SQLite3 handling, the SPL, and PDO PostgreSQL support. As always, you can download this pre-release from the PHP QA site for source downloads or Windows QA site for the Windows binaries.