Make sure you installed Hombrew PHP with --with-mssql
You can check using a command like below:
/usr/local/opt/php53/bin/php -i | grep Configure
When I pointed my apache to the homebrew version of php, it was already configured to have the mssql and freetds libraries working, otherwise you may have to add the .so file in the freetds directory to your php.ini file
Then I just had to something like this to my freetds.conf:
[my_mssql_server]
host = 127.0.0.1
port = 5433
client charset = UTF-8
tds version = 8.0
and configure codeignitor database.php
$db['default']['hostname'] = 'my_mssql_server';
$db['default']['username'] = 'sql_server_user';
$db['default']['password'] = 'sql_server_userpass';
$db['default']['database'] = 'SomeDbName';
$db['default']['dbdriver'] = 'mssql';
Reference: http://s4553711.github.io/post/2012-10-25/2012-10-25-PHP-with-MSSQL.html
To get it working on ubuntu, this was the last command I used to get it working (there were other packages installed before with may have helped)
sudo apt-get install libsybdb5 freetds-common php5-sybase