Declares multisite init_site methods as static

[walterebert]

In a multisite setup on PHP 8 initialisation methods cause a fatal error when adding/deleting a site. Declaring these methods as static fixes the problem.

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($function) must be a valid callback, non-static method Statify_Install::init_site() cannot be called statically

[sonarqubecloud]

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0.0% Duplication

[stklcode]

Thanks @walterebert for this fix.

The non-static declaration was probably never correct and did raise a warning with the latest PHP 7.x versions. Solutions looks reasonable.

Automated tests are not running for another reason, I will have a closer look at it later today.

[walterebert]

Unit tests for WordPress don't work with PHP 8 as far as I know. PHPUnit Polyfills might help, but I haven't used it yet: https://github.com/Yoast/PHPUnit-Polyfills

[stklcode]

I'm aware of the restrictions due to the PHPUnit 7 limit and our test cases do run fine with PHP 8 as long as you install with --ignore-platform-req=php. Current reason is the travis-ci.org to travis-ci.com migration that has not been done yet for this project, because we actually wanted to move to GH actions - the corresponding PR has not been merged either, so there's no CI at the moment.

[stklcode]

LGTM