bumb.php

<?php
if (empty($parent) || empty($tplOuter)){
    $parent = $modx->resource->get('id');
    $tplOuter = 1;
}

$letters = $modx->runSnippet('pdoResources', [
    'parent' => $parent,
    'tpl' => '@INLINE [[+$pagetitle:first]]',
    'sortby' => '{"pagetitle":"ASC"}',
    'limit' => 0,
    'outputSeparator' => '|'
]);

$arr = explode('|', $letters);
$letterArr = array_unique($arr);

foreach ($letterArr as $char){
    $list = $modx->runSnippet('pdoResources', [
       'parent' => $parent,
       'where' => '{"pagetitle:"ASC"}',
       'limit' => 0
    ]);

    $out .= $modx->getChunk($tplOuter, [
       'char' => $char,
       'items' => $list
    ]);
}
return $out;