Trouble with DUO_PHP package from GITHub

Hello
I have done a clone and install of the duo_php package from GITHub.
https://github.com/duosecurity/duo_php

I have setup a WEB SDK application in my DUO applications. I have copied the IKEY, SKEY, Host and created an AKEY and inserted them into duo_php\demos\simple-v2\index.php as per the instructions on GITHub.

When I try to run the above php page on my demo server, I receive the following error…

Fatal error : Uncaught Error: Class ‘DuoAPI\Frame’ not found in C:\xampp\htdocs\duo_php\demos\simple-v2\index.php:35 Stack trace: #0 {main} thrown in C:\xampp\htdocs\duo_php\demos\simple-v2\index.php on line 35

This is line 35 in the PHP file

$client = new \DuoAPI\Frame(
IKEY,
SKEY,
HOST,
null,
true,
\DuoAPI\SIGNATURE_CANON_JSON_STRING_BODY
);

Besides this question, what more do I need to do to make this authenticate with DUO.

Thanks for the help
Tony