php showing source code in localhost

I can think of 3 possibilities:

1. You don't have PHP installed. In that case run:

   sudo apt-get install php php-mysql
   

2. You have PHP installed but not enabled. You can try running:

   sudo a2enmod php7  
   sudo service apache2 restart  
   

3. You have PHP installed and enabled but you haven't restarted apache.

   sudo service apache2 restart
   

you need to install libapache2-mod-php sudo apt-get install libapache2-mod-php it is no longer pulled by other packages as a dependency in 14.04 it named libapache2-mod-php5, in 16.04 they dropped the number at the end for a meta package

After Upgrading to Ubuntu 17.10 this morning my answer was a combination of Sledge Hammer's and GM-Script-Writer-62850's answer. The exact commands I needed were:

sudo apt-get install libapache2-mod-php
sudo a2enmod php7.1
systemctl restart apache2