Each of the empty classes helps us simulate a PHP script that requires additional
external PHP files to use within the main script. By excluding any additional function
calls, we focus on the loading of the files using require_once().
Place each of the classes into separate files, ClassA.php, ClassB.php, ClassC.php,
ClassD.php, and save the files next to the code shown in Listing 3–3. Restart your web
server, and using the ab tool covered in Chapter 1, run the ab command shown in Listing
3–5.