Installing the php5-mssql package in Ubuntu is not as obvious as it should be. I was unable to find it in Synaptic nor apt-get at first. From the install it appears that the package php5-mssql is a virtual package, which really points to php5-sybase. To install php5-mssql for Ubuntu note that there are two methods.
Method 1 : Open up Synaptic Package Manager, search for and install "php5-sybase"
Method 2 : Open up a terminal and run:
aptitude install php5-mssql
If you read the progress in the terminal you will see it refer to the php5-sybase package and indicates that php5-mssql is simply a virtual package. You might notice too that the freetds package is also installed. After my install I also had to restart apache. Simple as that!