I installed Siteground Optimiser plugin (to switch to HTTPS but that’s another story). This checks that your site is compatible with the level of PHP recommended by Siteground and switches to it too. When I looked into this in December the recommended level was 7.0. The WP Simple Booking Calendar plugin has a few errors in the compatibility check but I emailed them and they said it was safe to upgrade to PHP 7.0. I couldn’t do this from the plugin (as it was showing the errors) but I was able to do it in the Siteground Cpanel area.
I’ve just looked at it now and the plugin is checking against PHP 7.1 now. Again there are errors in the calendar plugin but I expect that’s just the same as last time as that plugin doesn’t get updated.