[WordPress] Add extra buttons to WordPress editor (TinyMCE)

wp-extra-buttons.php

<?php
/**
 * Additional buttons in TinyMCE editor
 *
 * @param $buttons
 * 
 * @return array
 */
function wp_extra_buttons( $buttons ) {
    $buttons[] = 'hr';
    $buttons[] = 'sub';
    $buttons[] = 'sup';
    $buttons[] = 'fontselect';
    $buttons[] = 'fontsizeselect';
    $buttons[] = 'styleselect';
    $buttons[] = 'backcolor';
    $buttons[] = 'image';
    $buttons[] = 'media';
    $buttons[] = 'anchor';
    $buttons[] = 'wp_page';
    $buttons[] = 'cut';
    $buttons[] = 'copy';
    $buttons[] = 'paste';
    $buttons[] = 'code';
    $buttons[] = 'cleanup';
    
    return $buttons;
}

add_filter( 'mce_buttons_3', 'wp_extra_buttons' );

wp-tinymce-external-plugin.php

<?php
/**
 * External plugins for TinyMCE Editor
 *
 * @param $plugin_array
 *
 * @return array
 */
function wp_mce_external_plugins( $plugin_array ) {
    $plugin_array['print'] = get_template_directory_uri() . '/js/admin/editor_plugin.js';

    return $plugin_array;
}

add_filter( 'mce_external_plugins', 'wp_mce_external_plugins' );