Add custom tags to TinyMCE in WordPress

add_custom_xml_tags_to_tinymce_wordpress.php

<?php
/*
 * Our client needed some data sent in XML tags from Gravity Forms notifications to their system. These tags were being stripped by
 * TinyMCE when the client would switch from HTML to Visual. This was my fix for it.
 * 
 */

function sennza_add_custom_tinymce_tags( $init ) {
    // Command separated string of extended elements
    $ext = 'name[*],phone[*],src[*],ip[*],notes[*],email[*]';

    // Add to extended_valid_elements if it alreay exists
    if ( isset( $init['extended_valid_elements'] ) ) {
        $init['extended_valid_elements'] .= ',' . $ext;
    } else {
        $init['extended_valid_elements'] = $ext;
    }

    // Super important: return $init!
    return $init;
}

add_filter('tiny_mce_before_init', 'sennza_add_custom_tinymce_tags');