PHP 7.2 is currently in development and scheduled for November or December 2017 release. But, SiteGround has already included Release Candidate version for testing purposes on all their servers.
PHP 7.0 and 7.1 were significant performance updates for the PHP, and PHP 7.2 also continues with some performance updates, but, it will also bring many new features, improvements and language cleanup. The most important new feature is the inclusion of LibSodium library for modern encryption, decryption, hashing and passwords implementation.
SiteGround now includes the latest RC (release candidate) version of PHP 7.2 on all the servers, for all hosting packages. And, if you are not sure if you want to use PHP while in development, you can try it out only on the staging servers (most hosting packages include them). You can read more about this on the SiteGround blog here.
