According to the Health Check plugin:
PHP Version 5.5.38 - Your version of PHP, 5.5.38, is very outdated and no longer receiving security updates. You should contact your host for an upgrade, WordPress recommends using PHP version 7.2.
Wordpress 5.1.1 requires that php be updated to a more current version:
Upon testing of some of the features in a theme I'm testing, I'm sure I caused some errors into the logs.....
From the wp-admin dashboard:
WordPress has detected that your site is running on an insecure version of PHP...