Convert the Singleton class to a trait for PHP 8.1. compatibility

[johnwatkins0]

Description of the Change

As of PHP 8.1.0, when a method using static variables is inherited (but not overridden), the inherited method will now share static variables with the parent method. This means that static variables in methods now behave the same way as static properties. [Source]

Because of this change, the static $instance in the Singleton abstract class's instance method was going to be shared between all classes that inherit from Singleton. As a result, the instance would be set to the first class whose instance method is called, and none of the other classes would be created.

This converts the class to a trait, which essentially gives each Singleton class its own instances of the instance static property and method.

Also included: If you review the instance method previously in the abstract class and now in the trait, it runs its class's setup method if that method exists. That method was also being run in the plugin entry file as classes were instantiated, so it was being run twice.

Closes #117

How to test the Change

Check out this branch. Confirm all functionality works as expected.

Changelog Entry

Added - New feature Changed - Existing functionality Deprecated - Soon-to-be removed feature Removed - Feature Fixed - Bug fix Security - Vulnerability

Changed - Converted the Singleton abstract class into a trait for PHP 8.1. compatability.

Credits

Props @username, @username2, ...

Checklist:

• [ ] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.

[tlovett1]

This all looks good to me @claytoncollie @johnwatkins0 . My only question is what is the minimum version of PHP this will support?

[johnwatkins0]

@tlovett1 Traits were introduced in PHP 5.4, so this might be problematic since this plugin's README says it supports 5.3. I'll revisit and see what can be done while still working with 5.3.

[tlovett1]

@johnwatkins0 5.4 is fine. I'll update the readme. Thank you!