Using `var` or only `static` to declare a class property is PHP 4 code.
This updates the codebase to use explicit visibility modifiers introduced in PHP 5.
Props jrf.
Fixes#51557. See #22234.
git-svn-id: https://develop.svn.wordpress.org/trunk@49184 602fd350-edb4-49c9-b593-d223f7449a82