This release introduces new features from the Site Health project. WordPress will now detect if a site is running on an insecure, outdated version of PHP and display a notice in the dashboard with information about how to update PHP. It also includes checks for PHP version compatibility with plugins.

WordPress 5.1+ will prevent users from installing plugins that require newer versions of PHP than they have running.

This release also introduces a medley of miscellaneous improvements under the hood for developers, including (but not limited to) the following:

• New database table to store metadata associated with multisite networks
• Updated Cron API with new functions to assist with returning data, new filters for modifying cron storage
• New JavaScript build processes
• Updates to values for the WP_DEBUG_LOG constant
• Improved taxonomy metabox sanitization

When browsing plugins to install, WordPress 5.1 will display a warning for those plugins that require a higher PHP version than the one currently active. While that screen previously already included such information about WordPress version compatibility, it now does the same for PHP. Furthermore, for both of these potential issues, WordPress will now enforce these requirements, disabling the button to install such plugins.

This is only a first step in enforcing version requirements of plugins and themes. In a future WordPress version, these restrictions will expand also to updating or activating plugins and eventually cover themes as well.