Get All WordPress User Meta Keys

user_meta_key.php

<?php

/**
* Returns all unique meta key from user meta database
*
* @param no parameter right now
* @retun std Class
* @todo do what you do for each meta key.
*/
function get_user_meta_key() {

    global $wpdb;

    $select = "SELECT distinct $wpdb->usermeta.meta_key FROM $wpdb->usermeta";

    $usermeta = $wpdb->get_results($select);
  
    return $usermeta;
}