<?php

require("app/bootstrap.php");
$bootstrap = MagentoFrameworkAppBootstrap::create(BP, $_SERVER);
$obj = $bootstrap->getObjectManager();

// Set the state (not sure if this is neccessary)
$state = $obj->get('MagentoFrameworkAppState')->setAreaCode('frontend');

// Get cart items
$quote      = $obj->get('MagentoCheckoutModelSession')->getQuote();
$quoteItems = $quote->getAllVisibleItems();

echo "Found " . count($quoteItems) . " cart items";

// Expected output:
// Found 3 cart items

// Actual output:
// Found 0 cart items