- Removed `mixed` function return type from some functions for backward compatibility with php 7.4
- Refactored string concatination for readability.
- Added TODO for later
- Corrected PHPdocs
- Removed unneeded TODO
- Refactored function names with mixed snake and pascal case names
- Added missing PHPdocs
- Corrected Syntax
- Added noinspection parameters to PHPdoc for AJAX called functions
- Added missing function return types
- Added missing parameter types
- Added public keyword to functions without visibility modifier
- Corrected incorrectly formatted PHPdocs
- Added public to constants and functions missing a visibility keyword
- 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
