- Replaced TRUE/FALSE constants with true/false keywords
- Replaced NULL constant with null keyword
- Replaced `<?php echo` in views with shortened `<?=`
- Added missing variable declaration
- Added missing function return type in declaration
- replaced `== true`, `== false`, `=== true` and `=== false` in if statements with simplified forms
- when the payments array was folded into sale_data there was an earlier payments[] reference in the foreach loop that didn't get folded in.
- Update PHPdoc
- Added ::class to remove polymorphic call warning
- Removed unreachable 'break;' statement after return statement.
- Added missing return type
- fixed missing assignment of mailchimp_api_key
