The official PHP.net site has officially posted about the latest (alpha) release in the PHP 7.x series: PHP 7.1.0 Alpha 1:
The PHP development team announces the immediate availability of PHP 7.1.0 Alpha 1. This release marks the beginning of the first minor release in the PHP 7.x series. All users of PHP are encouraged to test this version carefully, and report any bugs and incompatibilities in the bug tracking system.
This is an alpha release and is not meant to be used in production. Features in this release include nullable types, allow specifying keys in list() usage and class constant visibility modifiers. You can see the full list of changes (and bugs fixed) in the NEWS file. As with all non-stable releases, you can get this latest alpha from this downloads page (for source) or the windows QA site (for the Windows binaries).