CodeIgniter is MVC based PHP framework, CodeIgniter in possible to multiple database connections.

Open the Database.php file

if you want to connect multiple databases in CodeIgniter then open the database.php file. this configuration file available in the array format so you have to do it second database in an array configuration.

there is two connection of the following example for CodeIgniter. first is by default CodeIgniter provide and we will create a second connection. change the database name, database username, and database password in the second connection.

see below example

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

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

// codeigniter provide default code
$db['default'] = array(
	'dsn'	=> '',
	'hostname' => 'localhost',
	'username' => '',
	'password' => '',
	'database' => '',
	'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
);

// second connectivity of database
$db['second_db'] = array(
	'dsn'	=> '',
	'hostname' => 'here add second database of hostname',
	'username' => 'here add second database of username',
	'password' => 'here add second database of password',
	'database' => 'here add second database of database name',
	'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
);

?>

The databases configuration after you can access the database.

//access default database
$this->load->database();
$query = $this->db->query('select * from register');

//access the second database
$second_db= $this->load->database('second_db', TRUE);
$query = $second_db->get('select * from register2');
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