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If you are the maintainer of this site, please check your database settings in the
settings.php file and ensure that your hosting provider’s database server is running. For more help, see the handbook, or contact your hosting provider.
There are two things you need to check.
1. Is mysqli is installed or not?
2. Is server able to connect database?
Is mysqli installed or not:
Check info.php file is mysqli is listed in this page, if not then your server will support only mysql not mysqli. If your host doesn’t have mysqli installed, you can it change to use mysql. (MySQLi stands for MySQL Improved.)
Open settings.php change $db_url from mysqli to mysql (settings usually in /sites/default-folder in your drupal installation).
$db_url = ‘mysqli://user:password@hostname/table’;
$db_url = ‘mysql://user:password@hostname/table’;
Is server able to connect database?
Check whether you have passed right username, password and database name and also check whether you have given full permission to access the database from the server.
Hope this will be useful for those having similar problems.