Why are my PHP files showing as plain text?

You'll need to add this to your server configuration:

AddType application/x-httpd-php .php

That is assuming you have installed PHP properly, which may not be the case since it doesn't work where it normally would immediately after installing.

It is entirely possible that you'll also have to add the php .so/.dll file to your Apache configuration using a LoadModule directive (usually in httpd.conf).

You should install the PHP 5 library for Apache.

For Debian and Ubuntu:

apt-get install libapache2-mod-php5

And restart the Apache:

service apache2 restart