To connect to MSSQL using PHP 5.6 we'll need to use PDO's DBLIB (PDO_DBLIB) http://php.net/manual/en/ref.pdo-dblib.php and FreeTDS http://www.freetds.org/
This quick tutorial is using Ubuntu Server 14.04
First, make sure you're up to date on all of your packages.
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
I'm using PHP 5.6.9 (https://launchpad.net/~ondrej/+archive/ubuntu/php5-5.6)