For some reason, the very first line breaks it. I've followed the instructions exactly, I updated and ran the composer file, and made the file called "charge-credit-card.php" and put in my ID and transaction key.
I am running php 5.6.40, which they say is high enough but I'm suspicous that could be an issue. When commenting out lines, I find that the very first line breaks the page, but some later ones are okay. the first line being " require 'vendor/autoload.php'; "
Can anyone think of reasons why this would not work? Or any other steps required to use the Authorize.net API?