This error occurs when you are using PDO to connect with MySQL database server and pdo_mysql extension is not enabled/installed in your server.
if you are working in localhost or dedicated server then you cab enable pdo_mysql extension yourself. You can enable this extension from php.ini file. Generally in linux system php.ini file is path is : /etc/php5/apache2/php.ini.
To enable pdo_mysql extension in shared hosting serve you need to contact hosting server support.