I've just been through the process of upgrading four Moodle sites to version 3.5. As ever I started with the least critical and worked through to the most critical. The last site has failed. A bit of diagnosis indicates that this is because this one site uses an external database to handle logins. The site was originally using Moodle 3.4.2
The whole thing is made more mildly more complicated by the fact that the first step was to upgrade PHP to version 7.
So, I completed the PHP upgrade.
Installed version 3.5 of Moodle, copied across the config files and the upgrade proceeded as expected.
At the end I attempted to login and turn off Maintenance Mode. The login fails with the error:
Call to undefined function mysql_connect()
A bit of research pointed me to https://moodle.org/mod/forum/discuss.php?d=346155
and the suggestion that I could edit the moodle database
A quick search through the database led me to mdl_config_plugins and enrol_database : dbtype changing (Value) to mysqli and purging cache solved the login issue.
Sadly, so far this hasn't changed anything.
I'm hoping that someone else has faced this issue...