In an announcement on the main PHP.net site, the PHP development team as officially announced the release of the first beta for PHP 7.2.0 - beta 1.

The PHP development team announces the immediate availability of PHP 7.2.0 Beta 1. This release is the first beta for 7.2.0. All users of PHP are encouraged to test this version carefully, and report any bugs and incompatibilities in the bug tracking system.

[...] PHP 7.2.0 Beta 1 builds on previous releases with: the much anticipated Sodium extension, Opcache improvements, Countable support for DOMNodeList and DOMNamedNodeMap improved handling for invalid UTF8 in json_decode() and many bugfixes...

You can find the complete list of updates in the NEWS entry for this beta and, as always, you can download this preview release from the QA downloads page or from windows.php.net/qa for the Windows binaries.