This is a proof of concept for using MODX 2.x as a dependency in a Slim application

src-Middleware-MODSlim.php

<?php
/*
 * This file is part of the MODSlim package.
 *
 * Copyright (c) Jason Coward <jason@opengeek.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace MODX\Slim\Middleware;


use Slim\Middleware;

class MODSlim extends Middleware
{
    /**
     * Call
     *
     * Perform actions specific to this middleware and optionally
     * call the next downstream middleware.
     */
    public function call()
    {
        $this->app->container->singleton('modx', function() {
            define('MODX_CORE_PATH', $this->app->config('modx.core_path'));
            define('MODX_CONFIG_KEY', $this->app->config('modx.config_key') ?: 'config');

            require MODX_CORE_PATH . 'model/modx/modx.class.php';

            $modx = new \modX('', $this->app->config('modx.config') ?: []);

            return $modx;
        });

        $this->next->call();
    }
}

www-index.php

<?php
$startTime = microtime(true);

require __DIR__ . '/../vendor/autoload.php';

$config = [
    "debug" => false,
    "https.port"=> 443,
];

if (is_readable(__DIR__ . '/../config.php')) {
    $loaded = include __DIR__ . '/../config.php';
    if (is_array($loaded)) {
        $config = array_merge($config, $loaded);
    }
}

$app = new \Slim\Slim($config);

$app->add(new \MODX\Slim\Middleware\MODSlim());

$app->error(function(\Exception $e) use ($app) {
    $app->getLog()->error($e->getMessage());
    $app->halt(500, $e->getMessage());
    $app->stop();
});

require __DIR__ . '/routes.php';

$app->run();

www-routes.php

<?php
$app->notFound(function() use ($app) {
    /** @var \modX $modx */
    $modx = $app->container->get('modx');
    $modx->initialize($app->config('modx.context') ?: 'web');

    $_GET[$modx->getOption('request_param_alias', null, 'q')]
        = $_REQUEST[$modx->getOption('request_param_alias', null, 'q')]
        = ltrim($app->request->getResourceUri(), '/');

    $modx->handleRequest();
});

$app->get('/', function() use ($app) {
    /** @var \modX $modx */
    $modx = $app->container->get('modx');
    $modx->initialize($app->config('modx.context') ?: 'web');

    $modx->resource = $modx->getObject('modResource', (int)$modx->getOption('site_start'));
    $modx->resourceIdentifier = $modx->resource->get('id');
    $modx->elementCache = array();
    $resourceOutput = $modx->resource->process();
    $modx->parser->processElementTags('', $resourceOutput, true, false, '[[', ']]', array(), 10);
    $modx->parser->processElementTags('', $resourceOutput, true, true, '[[', ']]', array(), 10);

    echo $resourceOutput;
});