Stand alone script to reindex Magento 2 - https://keithgreer.dev/magento-2-reindex-script-indexer

indexer.php

<?php

use Magento\Framework\App\Bootstrap;
include('app/bootstrap.php'); // Update as required, this line will work if placed in the Magento Root Directory
$bootstrap = Bootstrap::create(BP, $_SERVER);
$objectManager = $bootstrap->getObjectManager();


$params = $_SERVER;
$bootstrap = Bootstrap::create(BP, $params);
$obj = $bootstrap->getObjectManager();

$state = $obj->get('Magento\Framework\App\State');
$state->setAreaCode('frontend');

$indexer = $obj->get('Magento\Indexer\Model\Indexer\CollectionFactory')->create();
$ids = $indexer->getAllIds();

?><div style='top:45%; position:absolute; width:75%; text-align:center; color:#666; font-weight:bold; font-size:1.8em; '><?php

?><span style='color:green'>&#9679; Reindexing... </span> <?php

foreach ($ids as $indexerId) {
    $indexer = $obj->get('Magento\Framework\Indexer\IndexerRegistry')->get($indexerId);
    $indexer->reindexAll($indexerId); // this reindexes all
    echo '&block;';
}
echo ' Complete. ';