Get alphabetical Letters from MODX PDO Collection

alphabeticalLetters.php

<?php
/**
 * alphabeticalLetters
 *
 * DESCRIPTION
 *
 * This Snippet return alphabetical Letters from Collection
 *
 * PROPERTIES:
 *
 * &class required
 * &tpl required
 *
 * USAGE:
 *
 * [[alphabeticalLetters?
 *    &class=`glossary`
 *    &index=`title`
 *    &tpl=`@INLINE <li>[[+title]]</li>` ]]
 *
 */

# vars
$inlineTplPrefix = "@INLINE";
$tpl = $modx->getOption("tpl", $scriptProperties, $inlineTplPrefix . " ");
$class = $modx->getOption("class", $scriptProperties, false);
$index = $modx->getOption("index", $scriptProperties, "title");
$pdo = $modx->getService("pdoFetch");
$placeholders = [];

# get items
$items = $pdo->getCollection(
    $class,
    [],
    [
        "select" => "id, " . $index,
        "sortby" => '{"' . $index . '":"ASC"}',
        "limit" => 0,
    ]
);

# get letters
foreach ($items as $item) {
    $letter = strtoupper(substr($item[$index], 0, 1));
    if (!$placeholders[$letter]) {
        $placeholders[$letter] = [
            "id" => $item["id"],
            "letter" => $letter,
        ];
    }
}

# return
$output = [];
if (substr($tpl, 0, strlen($inlineTplPrefix)) == $inlineTplPrefix) {
    $chunk = $modx->newObject("modChunk", [
        "name" => "{tmp}-{" . uniqid() . "}",
    ]);
    $chunk->setCacheable(false);
    $tpl = str_replace($inlineTplPrefix, "", $tpl);
    foreach ($placeholders as $placeholder) {
        $output[] = $chunk->process($placeholder, $tpl);
    }
} else {
    foreach ($placeholders as $placeholder) {
        $output[] = $modx->getChunk($tpl, $placeholder);
    }
}

return implode("", $output);