codeigniter : Use 2 databases and hosted website - codeigniter-3

Question

I'm facing a very tough difficulty since a day ago, i still can't find any solutions regardless all my researches. My hosted website on bluehost has 2 databases, the first one is a mysql database which i connect to database easily and a sql database which i can't connect to it. I learn that i should have extension=php_pdo_sqlsrv_55_nts.dll, extension=php_sqlsrv_55_ts.dll in php.ini according my php version.
If my website is hosted how can i connect to the second sql database ?
My database file is like this :
<?php
defined('BASEPATH') OR exit('No direct script access allowed');
$active_group = 'default';
$query_builder = TRUE;
$db['default'] = array(
'dsn' => '',
'hostname' => 'localhost',
'username' => 'ausername',
'password' => 'apassword',
'database' => 'adatabase',
'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
);
$db['second_database'] = array(
'dsn' => '',
'hostname' => 'aaa.bbb-east-1.rds.amazonaws.com',
'port' => '14733',
'username' => 'aa',
'password' => 'apassword1',
'database' => 'adatabase1',
'dbdriver' => 'sqlsrv',
'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
);
When i load the page i have this error :
Fatal error: Call to undefined function sqlsrv_connect() in /home2/admedica/public_html/admedicall_v1/system/database/drivers/sqlsrv/sqlsrv_driver.php on line 144