The PHP.net site has announce the latest alpha release for the upcoming PHP 7.2.0 release: PHP 7.2.0 Alpha 2:
The PHP development team announces the immediate availability of PHP 7.2.0 Alpha 2. This release contains fixes and improvements relative to Alpha 1. All users of PHP are encouraged to test this version carefully, and report any bugs and incompatibilities in the bug tracking system.
[...] For information on new features and other changes, you can read the NEWS file, or the UPGRADING file for a complete list of upgrading notes. These files can also be found in the release archive.
As these are just development releases, the downloads for them are on the QA sites: source packages and Windows binaries. The final alpha for PHP 7.2.0 (third) will be published July 6th. Next comes the beta stage then Release Candidates. You can see the full release schedule plan here.