Best Practices for MailChimp Signup Form via API using PHP and AJAX

I had trouble finding working examples of a robust signup form for MailChimp using the MailChimp API with PHP. First, it wasn’t clear how to tag your users with any useful information when they sign up. Second, it wasn’t clear how to update a user’s information if they’ve already signed up in the past (not including all the examples out there that just plain didn’t work). So…