Old syntax for variables inside WP_User class

[prosti]

Inside the wp-includes/class-wp-user.php you can find

<?php
var $filter = null;

while all other class fields are either private or public according to the new PHP5 syntax.

​https://github.com/WordPress/WordPress/blob/master/wp-includes/class-wp-user.php#L90-97

If possible make the filter also public, because this is the equivalent of var.

[SergeyBiryukov]

Related: [28531]

[SergeyBiryukov]

In 39735:

Docs: Correct @access entry for WP_User::filter property.

Add missing @since entries for WP_User::back_compat_keys and WP_User::init().

Props keesiemeijer, prosti.
Fixes #39502, #39278.