For x86 win download http://www.microsoft.com/en-us/download/details.aspx?id=20098
for using with:
$db['default']['dbdriver'] = 'sqlsrv';
use the php_sqlsrv_XX_ts_vcX.dll in php.ini extension
if your webserver and your database server are not on same server with your app, install this:
http://go.microsoft.com/fwlink/?LinkID=188400&clcid=0x409
and for linux as webserver:
$db[‘default’][‘dbdriver’] = ‘mssql’;
download & install = php5-sybase + freetds
<br/>edit /usr/local/etc/freetds.conf
<br/>create pdo_dblib.so & php_mssql.so (if not allready exist on your extension dir), and apply to php.ini
sudo /etc/init.d/apache2 restart
and just dont forget to allow connection to your DB (default port for mssql is 1433)