Have had the same issue today after migrating the a site from a server running PHP5.6 to PHP7.0 (Obviously mysql_connect is not longer supported in PHP7.0) but the error persisted even after altering the drive in config.php to "mysqli" meaning users could not login.
Although the config.php had the database driver set to "Mysqli" the Database enrolment plugin had its own driver setting which was set to "mysql" and overriding the config.php setting.
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.
MDL-56933 talks about the issue a little more
Paul
HowToMoodle