PHP is an open source web scripting language that is widely use to build dynamic webpages.
To install PHP, open terminal and type in this command.
sudo apt-get install php5 libapache2-mod-php5 php5-mcrypt
After you answer yes to the prompt twice, PHP will install itself.
It may also be useful to add php to the directory index, to serve the relevant php index files:
sudo nano /etc/apache2/mods-enabled/dir.conf
Add index.php to the beginning of index files. The page should now look like this:
<IfModule mod_dir.c>
DirectoryIndex index.php index.html index.cgi index.pl index.php index.xhtml index.htm
</IfModule>