The official PHP site has posted an announcement of the release of the latest alpha for the upcoming PHP 7.3.0 release: PHP 7.3.0 alpha 4.
The PHP team is glad to announce the release of the fourth PHP 7.3.0 version, PHP 7.3.0alpha4. The rough outline of the PHP 7.3 release cycle is specified in the PHP Wiki.
This is a development release and should not be used in production. Changes in this alpha release include updates to core functionality, updates to filtering, FPM handling, mbstring and zip archive functionality. You can check out the NEWS file for a complete list of updates.
To download this latest release to try with your codebase, you can download the source from the QA downloads or get the Windows binaries from the Windows QA site. The next planned release of PHP 7.3.0 is the first beta and is set to release on August 2nd, 2018.