The PHP.net site has posted an official announcement about the release of the latest Release Candidate in the PHP 7.1.x series: PHP 7.1.0 Release Candidate 4:

The PHP development team announces the immediate availability of PHP 7.1.0 Release Candidate 4. This release is the fourth release candidate for 7.1.0. All users of PHP are encouraged to test this version carefully, and report any bugs and incompatibilities in the bug tracking system. [...] The fifth release candidate will be released on the 27th of October. You can also read the full list of planned releases on [our wiki](https://wiki.php.net/todo/php71).

As usually for Release Candidates you can grab the latest test build from this QA site (for source) and the windows.php.net QA site for the binaries. You can find out more about what's changed in this RC in the NEWS file and start walking through the upgrade to PHP 7.1.0 when it's released using the upgrade guide.

Reminder, this is a development preview not a stable release so do not use it in production!