Get gallery's cover from Template Variable

getGalleryCoverByTV.php

<?php
/**
 * getGalleryCoverByTV snippet
 * Get gallery's cover from Template Variable
 * 
 * @author  goldsky <goldsky@virtudraft.com>
 * @license GPLv3
 */

$toPlaceholder = $modx->getOption('toPlaceholder', $scriptProperties);
$modx->setPlaceholder($toPlaceholder, ''); // reset for snippet loop
$resourceId = $modx->getOption('resourceId', $scriptProperties);
$tv = $modx->getOption('tv', $scriptProperties);
if (empty($resourceId) || empty($tv)) {
    return;
}

$resource = $modx->getObject('modResource', $resourceId);
if (!$resource) {
    return;
}
$galleryId = $resource->getTVValue($tv);
if (empty($galleryId)) {
    return;
}

$gallery = $modx->getService('gallery', 'Gallery', $modx->getOption('gallery.core_path', null, $modx->getOption('core_path') . 'components/gallery/') . 'model/gallery/', $scriptProperties);
if (!($gallery instanceof Gallery)) {
    return '';
}
$modx->lexicon->load('gallery:web');

$galAlbum = $modx->getObject('galAlbum', $galleryId);
if (!$galAlbum) {
    return;
}

$cover = $galAlbum->get('cover_filename');
if (empty($cover)) {
    return;
}
$output = $modx->getOption('gallery.files_url') . $cover;
if (!empty($toPlaceholder)) {
    $modx->setPlaceholder($toPlaceholder, $output);
    return;
}
return $output;