Once I confirmed that both production and DR were available, I integrated both apps with Azure Active Directory so only my AD-authenticated users could access the site. I did this by configuring Authentication/Authorization for each of my two apps and used "Express Mode' for each. To test, I opened an Incognito tab in Chrome and browsed to my php script (http://mydomainname.com/demo.php). Before anything happened, I was prompted to login to my Azure account. Success!
Overall, this deployment took about a half hour from start to finish. Maybe not even that long.
My next project is going to expand on this one by load balancing each app (Production and DR) within each app's respective location. This will add an additional level of redundancy as well as performance load balancing. Keep an eye out for my next post on it.