The basic Ubunu server install does not have apache and php installed by default, so you will need to add them to the system.
sudo apt install apache2 php php-mysql
Verify that your webserver and php are working, by visiting you public ip address. This should display the default apache welcome page. Next, navigate to /var/www/html and create a new file that will display the php configuration and thus verify that php is working correctly.
cd /var/www/html sudo nano info.php
Content of the file:
<?php echo phpinfo(); ?>
Now open the info.php file in your web browser to verify php is working correctly.