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/cache.php

@@ -4,7 +4,7 @@
  * @group cache
  */
 class Tests_Cache extends WP_UnitTestCase {
-	var $cache = null;
+	public $cache = null;
 
 	function setUp() {
 		parent::setUp();