A need to move to Moodle 3.5 from v 3.3 necessitated an upgrade to php7. All worked well until the Moodle upgrade completed after which log in to the site failed with a mysql_connect() error.

The issue arose because the site used an external database for autheticating log-ins.

The site has been in use since the earliest days of Moodle so has been upgraded regularly. At no point did there appear any warning or suggestion that the authdb type should be switched to mysqli so it has sat in the background as a type mysql. The switch to php7 broke the external database authentication.

I suggest a warning to check this setting before a Moodle upgrade/update is performed where External Database authentication is in use.