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/includes/mock-mailer.php

@@ -3,7 +3,7 @@ require_once ABSPATH . 'wp-includes/PHPMailer/PHPMailer.php';
 require_once ABSPATH . 'wp-includes/PHPMailer/Exception.php';
 
 class MockPHPMailer extends PHPMailer\PHPMailer\PHPMailer {
-	var $mock_sent = array();
+	public $mock_sent = array();
 
 	function preSend() {
 		$this->Encoding = '8bit';