Connect to your Raspberry Pi.
If you havent set up ssh a guide can be found here: http://www.instructables.com/id/Use-ssh-to-talk-with-your-Raspberry-Pi/
Run the commands:
sudo apt-get update
sudo apt-get install apache2 php5 libapache2-mod-php5
Now your Raspberry Pi should display (if you open it in your browser): It works!
If you are uncertain of address type ifconfig
in your ssh-window. Should be 192.168.1.12 for example.