firstchildid.snippet.php

<?php
/**
 * FirstChildId
 * Gets the first child id of the given id. Works as output filter.
 * Example use: [[*id:FirstChildId]]
 *
 * @autor Bert Oost at OostDesign.nl <bert@oostdesign.nl>
 */

$id = (!empty($input)) ? $input : false;
if(!empty($id)) { $id = $modx->getOption('id', $scriptProperties, $modx->resource->get('id')); }

// select the first child
$c = $modx->newQuery('modResource');
$c->select('`modResource`.`id`');
$c->where(array(
    'parent' => $id,
    'published' => true
));
$c->sortby('menuindex', 'ASC');
$c->limit(1);

$child = $modx->getObject('modResource', $c);

if(!empty($child) && $child instanceof modResource) {
    return $child->get('id');
}

return $id;
?>