There are several causes for this error.  From CMS updates / upgrades (specifically with Joomla) to your host changing PHP settings or even versions on their side.  Luckily there is more than one workaround for this issue:

 

1. php.ini

 ; Default host for mysql_connect() (doesn't apply in safe mode).
mysql.default_host =

Change to:

; Default host for mysql_connect() (doesn't apply in safe mode).
mysql.default_host = localhost

2. configuration.php (Joomla)

 

 public $dbtype = 'mysqli';
change to:
public $dbtype = 'mysql';

3.  PHP Settings in cPanel

You can enable the mysqli setting for PHP.

Log in to cPanel, then click on SELECT PHP VERSION.

PHP_Ver_zps489ba912

 

On the next page, select PHP 5.5 and click ‘Set as current’ .  You will see various PHP settings, with mysqli being selected by default for this version.

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