OK! I have my new Hello World application set up in the Developers app. Now what?
You have your app all ready to go, but when you go to your URL (one like http://apps.facebook.com/jays-fantastic-demo), you get a 404 error message (see Setting Up a New App for more about setting up your app). The problem is that you don’t have anything on your server for Facebook to display, so let’s put something there. Make a new blank page in your favorite text editor (see Facebook Developer Software Toolbox for a list) and stick this in it:
<p>Hello <fb:name uid="loggedinuser" firstnameonly="false" useyou="false"/>!</p>
Save the file to your http://yourdomain.com server as index.php, and then reload your new Facebook app.
You should now have a fully functional Facebook application that only you can see. Congrats! This app can be used as a great test bed for all of your ideas as you work your way through the rest of this book.