Code Modernization: Use explicit visibility for class property declarations.

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

tests/phpunit/tests/theme/customHeader.php

@@ -4,7 +4,7 @@
  */
 class Tests_Theme_Custom_Header extends WP_UnitTestCase {
 
-	static $post;
+	public static $post;
 
 	protected static $header_video_id;