We don’t want to be mixing the Quickstart and the PHP example. They are two very different examples. In the Quickstart /payment is the function in the server.js file that process the payment. If your using the PHP example the index.php is the file where the form is rendered. The eventHandler in the PHP example is where tokenize is called and passed to the payment request.
If you spin up the PHP example as per the README.md it will work out of the box. You don’t need any of the code from the Quickstart. That Quickstart is a Node.js example. 