Error connecting to database - check your database connection properties in constants.php file

SQLSTATE[HY000] [2005] Unknown MySQL server host '_DB_HOST_' (-2)
PHP Version : 5.3.29

Catchable fatal error: Argument 1 passed to Functions::__construct() must be an instance of PDO, null given, called in /home/studio-graph/ftp/topczewo/admin/includes/Session.php on line 32 and defined in /home/studio-graph/ftp/topczewo/admin/includes/Functions.php on line 11