Codeigniter database connection error

Unable to connect to database in Codeigniter:

I have changed mysql to mysqli in database.php but unable to connect to database.

$active_group = 'default';
$query_builder = TRUE;

$db['default'] = array(
'dsn' => '',
'hostname' => 'localhost',
'username' => 'root',
'password' => 'administrator',
'database' => 'industry',
'dbdriver' => 'mysqli',
'dbprefix' => '',
'pconnect' => FALSE,
'db_debug' => (ENVIRONMENT !== 'production'),
'cache_on' => FALSE,
'cachedir' => '',
'char_set' => 'utf8',
'dbcollat' => 'utf8_general_ci',
'swap_pre' => '',
'encrypt' => FALSE,
'compress' => FALSE,
'stricton' => FALSE,
'failover' => array(),
'save_queries' => TRUE
);

A PHP Error was encountered

Severity: Warning

Message: mysqli::real_connect(): (HY000/2002): No connection could be made because the target machine actively refused it.

Filename: mysqli/mysqli_driver.php

Line Number: 201