FormIt custom validator. Checks if the username is already taken by another user

checkUsername.snippet.php

<?php
/**
 * checkUsername
 * FormIt custom validator. Check if no other user has the same username
 * 
 * Usage:
 *  &validate=`yourfield:checkUsername`
 * To set an custom error text; use like default ones;
 *  &vTextCheckUsername=`Your custom error text!`
 *
 *  NOTE: Add to FormIt with &customValidators=`checkUsername`
 * 
 * @author Jeroen Kenters <jeroen@kenters.com>
 */

//get logged in username (or empty when no logged in user)
$currentUser = $modx->getLoginUserName();

//if current user is same as posted value, proceed
if($currentUser == $value) {
    return true;
}
//not the same, so check if another user already has this username
$count = $modx->getCount('modUser', array('username' => $value));
if($count > 0) {
    $validator->addError($key, $validator->_getErrorMessage($key,'vTextCheckUsername','Username already taken!'));
    return false;
}
return true;