PHP 5.6 is the intended version from WordPress 5.1

Quick Highlights: #Quick Highlights:

• WordPress 5.1 will start showing notices to administrators of sites that run on long outdated PHP versions. (Below PHP 5.6
  version). See bump of the minimum required PHP version.

• Provided filter wp_update_php_url for a more dynamic approach on the code level. Replacing the URL should happen in cases where a more specific guide to update PHP on the given environment exists. E.g. screenshot:

• Fatal Error Protection with the so-called WSOD protection (white-screen-of-death protection), A mechanism has been implemented to detect fatal errors and, in certain designated areas of WordPress, recover from them. E.g. Screenshot.

Note that, while the primary reason for implementing the fatal error protection mechanism was making the process of updating PHP less “dangerous”, it is technically not tied to the update at all. In fact, it will be enabled permanently and discover fatal errors under any circumstances.

• The functionality can also be disabled entirely if that is preferred, via a new constant WP_DISABLE_FATAL_ERROR_HANDLER or, more dynamically, a corresponding wp_fatal_error_handler_enabled filter.

• WordPress 5.1 will display a warning for those plugins that require a higher PHP version than the one currently active.

Read more at https://make.wordpress.org/core/2019/01/14/php-site-health-mechanisms-in-5-1/